The Phrasing is Not a Direct Comment on Jesus being over 40 Years Old
Irenaeus is in error with his logic here, partly because he is missing the context and particular significance of the statement.
Rather than being a direct comment on Jesus' age (i.e. over 40 years old), the number 50 is stated because of its significance in Levitical service. Numbers chapter 4 shows the initiating of the Levites into service for the tabernacle care and rituals. Numerous times it is noted that those being put into place were those between 30-50 years of age (v.3, 23, 30, 35, 39, 43, 47). The lower limit of ability to serve actually was 25 years old, but none that young needed to be enlisted in the original placing (hence the numbering of 30-50). However, the upper limit was very fixed, and emphatically noted in Num 8:23-26, particularly v.25 (NKJV):
23 Then the LORD spoke to Moses, saying, 24 “This is what pertains to
the Levites: From twenty-five years old and above one may enter to
perform service in the work of the tabernacle of meeting; 25 and at
the age of fifty years they must cease performing this work, and shall
work no more. 26 They may minister with their brethren in the
tabernacle of meeting, to attend to needs, but they themselves shall
do no work. Thus you shall do to the Levites regarding their duties.”
There was still a ministry one could do after 50, but not the actual work of the tabernacle. Certain of their responsibilities in bearing the tabernacle in transit were no more needed once rest was gained under King David (1 Chr 23:25-32), but they were additionally given the specific duties of thanksgiving and praise by him (1 Chr 23:30, 2 Chr 7:6, 8:14, et. al.; cf. 1 Chr 28:13, 21).
It is helpful to note as well that the Levites held a special place with God, they were a special possession (Num 8:14, 18):
14 Thus you shall separate the Levites from among the children of
Israel, and the Levites shall be Mine. ... 18 I have taken the Levites
instead of all the firstborn of the children of Israel.
Why this background is significant becomes evident from the context of the discussion in John 8.
Relevant Context of John 8
In John 8, Jesus is speaking to the religious workers of His day, the Pharisees (e.g. v.13), who served in the temple area (v.20), along with probably a larger body of Jewish people, "the Jews" (v.22).
Jesus is declaring, in part in John 8, His service for God, not just as a worker, but as a Son (v.26, 28-29), as one with a special place in the Father's eyes, a place that those He was contending with did not have (v.37-38). They were not even doing the works of Abraham, who they claimed to be their father (v.39), much less the works of God, who they also claimed to be their father (v.41), but rather the works of the devil (v.44), and thus not serving or honoring God at all (v.47).
After Jesus asserts that keeping His word prevents one from death (v.51), these opponents think they have Him, for Abraham and the prophets are dead (v.52-53), who they believe quite obviously could not have heard His word, much less kept it. The incredible assertion Jesus then makes in v.56 is that:
Your father Abraham rejoiced to see My day, and he saw it and was
- implies Abraham had heard about Jesus' day (i.e., in context, at the least His ministry of God's word, as being discussed in John 8), for
- it states Abraham had foreseen Jesus' ministry, and
- states Abraham was happy for Jesus' work for the Father.
So the setup for v.57 is a contrast of works and service between the Jews and Jesus toward God, and the nature of the placement of the two groups as to which really had the special placement in relation to the Father. Upon Jesus' bold assertion about Abraham's knowledge, they then say:
you are not yet fifty years old, and have You seen Abraham?
The Greek is more directly:
Πεντήκοντα ἔτη οὔπω ἔχεις, καὶ Ἀβραὰμ ἑώρακας;
fifty years not yet you have and Abraham you have seen?
A few manuscripts replace ἑώρακας with εωρακεν σε1, rendering the last phrase then "Abraham has seen you?"
The point is clear with either rendering, Abraham and Jesus have particular knowledge of one another. How can this be? they ponder, since Jesus is less than fifty, i.e. Jesus is not even old enough to be considered retired from temple service under the Levitical law, much less old enough to know Abraham who is long dead.
The Pharisees knew the Law (the letter of it, anyway), and the reference is in relation to that. They themselves may not be Levites (the Pharisees were not necessarily so), but the allusion is to the only "retirement" age in Levitical law (at least, the only one I can think of off the top of my head), of which Jesus had not even attained that age yet, so how could He (how dare He!) make such an assertion as He does.
So the reference to 50 years old in John 8:57 is not to indicate Jesus is necessarily in His 40's, but rather a statement about the fact that He is not yet even old enough to be considered retired from working in the service of God in tabernacle/temple service,2 much less be old enough for Abraham to have seen His day.
Why Irenaeus Argues what He does of Jesus' Age
Peter Kirby has an article that walks through the cultural context of Irenaeus's time, showing that philosophical and cultural ideology drove his interpretation (though I do not believe that is Kirby's direct intent). His conclusion states (bolding added by me, along with extra notations in brackets given below):
Irenaeus was of sound mind when he wrote, in agreement with tradition
and scripture. Instead of stifling his voice, it is necessary to
elucidate the cultural context of the passage  and witness that the
five stages of life that he sees in Christ , culminating in an age of
death near 50 , is drawn straight from cultural assumptions about the
stages of life and the prime of life  that in his [Irenaeus'] day would be
commonplace, especially among those with an education in Greek.
 Referring to the Against Heresies 2.22.6 passage, part of which the OP quoted in the question.
 Kirby elucidates the seven stages of life in Greek philosophy, but of these five stages, he refers to Iranaeus' earlier statements in Against Heresies 2.22.4, where Kirby summarizes the stages of life in Iranaeus' thought as "infants, children, boys, youth, and old men," which Iranaeus believed Christ had to have passed through all the stages in order to sanctify and be an example for each stage of life.
 Kirby having earlier established 49, i.e. 7 x 7, was the philosophically recognized age of perfection and maturity of the mind.
 Irenaeus' day, not necessarily Jesus' day, though probably so even then among Greeks, but the context is that of very orthodox Jews within the Temple, rather than the among the Greek philosophers.
So one finds that Iranaeus had a combination of Greek philosophical and cultural background regarding the stages of life and prime age of life that he coupled with a preconceived theological backdrop of Christ needing to pass through all stages of life to fulfill His work sanctifying and exemplary work driving his exegetical understanding of the John 8:57 reference. This, coupled with a statement that on the surface is logical, that one would not mention 50 if the person were not in their 40's, does show, as Kirby notes, Irenaeus "was of sound mind when he wrote."
I just believe Iranaeus failed to consider the Jewish context of the actual event in which the statement took place, superseding it with a Greek context of his day.
Typical Evidences for Jesus in His 30's at His Death
The first part of Luke 3:23 is the starting point for most scholars holding that Jesus was somewhere near 30 years old at his crucifixion (NKJV/Majority Text):
Now Jesus Himself began His ministry at about thirty years of age...
αὶ αὐτὸς ἦν ὁ Ἰησοῦς ὡσεὶ ἐτῶν τριάκοντα ἀρχόμενος3
So it is fairly unambiguous that Luke's gospel attributes the start of Jesus' ministry work at "about" (ὡσεὶ) thirty years old (which, depending upon how loose an approximation one wants to consider that statement, means He was likely 29-32 years of age—just about to be 30, or close enough past 30 to use the round number of 30 to indicate his general age).
Note how this number, also, is associated with the Levitical priesthood service ages noted previously above. 30 was the common low end start, though 25 was the absolute earliest age.
There are only three Passovers that Jesus is noted to have attended during His ministry time (John 2:13; 6:4; 11:55), so these must be accounted for. Otherwise, the dating of Christ living only into his thirties is primarily based upon the views of when He was born, when His ministry began in relation to John the Baptist's ministry (which can be determined for its start date by Luke 3:1-3) and the most likely years of when Jesus died, given the historical facts that are known.
Harold Hoehner has a series of articles about a lot of these dating issues, and without necessarily agreeing with all his conclusions, the relevant evidence related to Irenaeus's view are chiefly these:4
- "the terminus a quo [i.e., earliest possible point] of Christ’s birth is difficult to pinpoint but it was probably taken sometime between 6 and 4 B.C." (Hoehner, "Part I," 348); he explains the various difficulties, but the key passage related to the earliest number is Luke 2:1-5, where there was a census being done at the time of Christ's birth, which census "first took place while Quirinius was governing Syria" (v.2). The dating of that specifically is where much of the question resides. A major part of Hoehner's "Part I" gives his reasoning for his probable dating of the earliest possible birth date for Christ as 6 B.C. Regarding the latest point, "Christ could not have been born later than March/April of 4 B.C" ("Part I," 340) because of the dating of Herod the Great's death, who was alive at Christ's birth (Mt 2:1; Lk 1:5).
- "He was crucified before Pilate’s departure from Judea in the winter of A.D. 36/37" (Hoehner, "Part III," 147), since Pilate was central to Jesus' crucifixion (e.g. John 19).
So 6 B.C. to 37 A.D. gives a maximum age of 43. This fits still with Irenaeus's view of Christ being in His 40's, but only if the extreme points are taken for His birth and death. Hoehner discusses the crucifixion dating in more detail, with the result of further narrowing factors being there are "only two plausible dates for the crucifixion, namely, A.D. 30 and 33" ("Part V," 338).
While Hoehner himself believes B.C. 5 to A.D. 33 is the most likely time frame for Jesus' life, if one accepts His argumentation generally, the evidence does not allow for Christ to have reached his 40's for the crucifixion, and hence, most scholars have viewed Irenaeus as being incorrect on this point.
1 Eberhard Nestle and Erwin Nestle, Nestle-Aland: NTG Apparatus Criticus, Barbara Aland, Kurt Aland, Johannes Karavidopoulos, Carlo M. Martini, and Bruce M. Metzger, eds., 28 rev. ed. (Stuttgart: Deutsche Bibelgesellschaft, 2012), notes on John 8:57. This is the only significant variant in the text here.
2 There are some who simply state that "Fifty years was with the Jews the completion of manhood" (quoting [Henry] Alford in Robert Jamieson, A. R. Fausset, and David Brown, Commentary Critical and Explanatory on the Whole Bible [1871; Oak Harbor, WA: Logos Research Systems, Inc., 1997]), and similarly a roughly contemporaneous quotation, "The fiftieth year was the full age of a man, Numb. 4:3" (quoting [August] Tholuck in John Peter Lange and Philip Schaff, A Commentary on the Holy Scriptures: John [1865–80; Bellingham, WA: Logos Bible Software, 2008]). I have not found a definitive statement that the Jews in fact believed this was the age for "the completion of manhood" (that sounds a bit 'final' to me), but such a view may have been an overstatement with respect to the Numbers chapter 4 information, since the quote from Tholuck includes a reference to Num 4:3.
3 Some readings omit the article before the name and some have the word order varied, hence the UBS/NA28 reading:
Καὶ αὐτὸς ἦν Ἰησοῦς ⸀ἀρχόμενος ὡσεὶ ἐτῶν τριάκοντα
These variations do not affect the meaning of the text at all.
4 Harold Hoehner, "Chronological Aspects of the Life of Christ" in five parts across three years/volumes of Bibliotheca Sacra:
- (1973) "Part I: The Date of Christ’s Birth," 130:338-351.
- (1974) "Part II: The Commencement of Christ’s Ministry," 131:41-54
- (1974) "Part III: The Duration of Christ’s Ministry," 131:147-162
- (1974) "Part IV: The Day of Christ’s Crucifixion," 131:241-264
- (1974) "Part V: The Year of Christ’s Crucifixion," 131:332-348
- (1975) "Part VI: Daniel’s Seventy Weeks and New Testament Chronology," 132:47-65