It's an interesting question, and one that has caught the eye of commentators for a long time. Let's get the text of Acts 7:58b first:
[NASB] ... and the witnesses laid aside their robes at the feet of a young man named Saul.
[SBL GNT] ... καὶ οἱ μάρτυρες ἀπέθεντο τὰ ἱμάτια αὐτῶν παρὰ τοὺς πόδας νεανίου καλουμένου Σαύλου
The term here is νεανίας [neanias], used only in Acts in the NT (7:58; 20:9; and 23:17), although none of these contexts gives much help for determining how youthful a neanias might be. It is also used about 30 times in the Septuagint (LXX),1 however, as well as more widely of course. In the LXX, it can sometimes simply refer to fighting men (2 Sam 6:1; 1 Kgs 12:21).
R.J. Knowling included a pithy but helpful comment on this problem in the old Expositor's Greek Testament (1897), vol. 2 pp. 201-202, including the observation that
Josephus applies the term to Agrippa I. when he was at least forty.
Jos., Ant., xviii., 6, 7. [+ Whiston]2
Knowling suggests the possible range for the term anywhere between 24 and 40, so roughly the boundaries suggested by OP's research. More recently, Craig Keener's (massive) Acts commentary reflects on the issue.3 He adds some consideration about the "phases of life" in both Roman and Jewish settings, all of which adds to the wooliness (not the clarity) of the lower and upper limits suggested by this term. Keener himself opts for a younger birth-date for Saul.4
In any case, it appears that "young man" in Acts 7:58 contrasts with "old man" on the broad scale of adult male life. The dates involved do not present a problem, as they might appear to in our contemporary West's youth-obsessed culture.
- Jdg. 16:26; 17:7, 11; 19:3, 9, 11, 13; Ruth 3:10; 1 Sam. 20:31, 37; 2 Sam. 6:1; 1 Ki. 12:21; 1 Chr. 19:10; 1 Es. 8:88; 2 Ma. 3:26, 33; 7:25, 30; 10:35; 12:27; 4 Ma. 8:5, 27; 9:13, 21, 25; 14:9; Prov. 7:7; 20:29; Zech. 2:8; and Dan. 1:10.
- I'm not sure that this is the correct reference; might need some adjustment in a future edit.
- Craig Keener, Acts: An Exegetical Commentary: 3:1-14:28 (Baker Academic, 2013), pp. 466ff..
- For random comparison, it's often noted that the study of Ezekiel 1 was restricted to men over thirty in b. Ḥagigah 13b -- another kind of age of "maturity"?