The operative Hebrew word in Isa 9:6 is אֲבִיעַ֖ד ('abi'ad) - a compound word of father + perpetuity/eternity (עַד BDB), eg, Gen 15:18, Isa 57:15, Ps 111:3, 10, 112:3, 112:9, 132:14, 19:10, etc.. As with such Hebrew compound words, one might validly translate this as:
- everlasting father (eg, NIV, BSB, ESV, etc)
- father of eternity (eg, YLT)
- eternal father (eg, NASB, CSB, CEV, etc)
However, I struggle to see a valid justification for "father of the coming age"; this appears to have been motivated by Ananasius' theology or similar; see Benson's comment below. Indeed, the thoughtful commentaries offer these remarks:
Pulpit -
The Everlasting Father; rather, Everlasting or Eternal Father. But here, again, there is a singularity in the idea, which makes the
omission of the article unimportant; for how could there be more than
one Everlasting Father, one Creator, Preserver, Protector of mankind
who was absolutely eternal? If the term "Father," applied to our Lord,
grates on our ears, we must remember that the distinction of Persons
in the Godhead had not yet been revealed.
Ellicott -
In “Father of Eternity,” (LXX. Alex. and Vulg., “Father of the age to
come “) we have a name which seems at first to clash with the
formalised developments of Christian theology, which teach us, lest we
should “confound the persons,” not to deal with the names of the
Father and the Son as interchangeable. Those developments, however,
were obviously not within Isaiah’s ken, and he uses the name of
“Father” because none other expressed so well the true idea of loving
and protecting government (Job 29:16, Isaiah 22:21). And if the
kingdom was to be “for ever and ever,” then in some very real sense he
would be, in that attribute of Fatherly government, a sharer in the
eternity of Jehovah. Another rendering of the name, adopted by some
critics, “Father (i.e., Giver) of booty,” has little to recommend it,
and is entirely out of harmony with the majesty of the context.
Benson -
The everlasting Father — Hebrew, אבי עד, The Father of eternity:
having called him a child and a son, lest this should be
misinterpreted to his disparagement, he adds that he is a Father also,
even the Father of eternity, and, of course, of time, and of all
creatures made in time. Christ, in union with the Father and the Holy
Ghost, is the God and Father of all things, the maker and upholder of
all creatures, John 1:3; Hebrews 1:3; and especially the Father of all
believers, who are called his children, (Hebrews 2:13,) and the author
of eternal life and salvation to them, Hebrews 5:9.