no longer able to do something at an acceptable level because of age.
The dancer was past her prime, though she performed occasionally as a guest artist.
past (someone's or something's) prime
No longer in an ideal state or condition. Many gymnasts are past their prime when they are only teenagers.This computer is so slow that it's definitely past its prime.
past someone's(or something's) prime
Fig. beyond the most useful or productive period. Joan was a wonderful singer, but she's past her prime now. This old car's past its prime. I'll need to get a new one.