past one's prime

be past one's prime (beyond the peak of one's powers) — пора (его) расцвета миновала; лучшие годы позади; уже не тот (каким был), ~песенка спета (контекст)

Example: Jean still plays tennis but at 89 she's obviously past her prime. (Answers.com)

see also
[over the hill]
[long in the tooth] (to do sth)
[no spring chicken]

prime of life