Louw-Nida's lexicon has the following for ὑπέρακμος:
"pertaining to being of an age beyond the prime of life (in 1 Cor 7:36 a reference to a woman beyond the normal marriageable age) 'past one's prime, past marriageable age'" (Semantic Domain 67.158).
While Louw-Nida's definition of ὑπέρακμος is quite plausible, it is good to keep in mind that the prefix ὑπέρ possibly refers to "intensity" which means that one could understand Paul's language as a reference to intense sexual desires.
"If someone with strong passions thinks that he is behaving badly towards his fiance'e and that things should take their due course, he should follow his desires. There is no sin in it; they should marry" (1 Cor. 7:36, New Jerusalem Bible).
I also found a substantial and relatively long discussion in Anthony C. Thiselton's commentary on the Greek text of The First Epistle to the Corinthians: he provides both pertinent diachronic and synchronic details concerning ὑπέρακμος. See Thiselton, 593-598.
Finally, Will Deming has produced a very detailed study of 1 Corinthians 7 entitled Paul on Marriage and Celibacy: The Hellenistic Background of 1 Corinthians 7. Cambridge: Cambridge University Press, 1995.