David, above, writes:

> 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. 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.

There is a lengthy treatment of Paul's youth and early career in Chapters 2 to 4 of *The Life and Epistles of St. Paul* by Rev. W J Conybeare and Very Rev J S Howson (first published 1856, and appears in many subsequent editions). In a [footnote on page 37][n], and referring to Acts 7 v 58, they write, simply: **"It must be remembered, however, that the term *νεανίας* was applied to all men under 40".**

Whilst adding that the year of Paul's birth is not known, they note (p. 37) that in a sermon attributed to St. Chrysostom, his year of birth was given as 2 BC. If that were correct, and (a) holding to the widely-accepted date of 33 AD for Christ's crucifixion and (b) placing the stoning of Stephen at say 35 AD, Paul would have been 36 or 37 years old on that occasion.   

REF: *The Life and Epistles of St. Paul* by Rev. W J Conybeare and Very Rev J S Howson (1898).

[n]: https://archive.org/stream/cu31924031189107#page/n63/mode/2up