past one's prime
Beyond the peak of one's powers, as in Jean still plays tennis but at 79 she's obviously past her prime. Also see the synonym over the hill; prime of life.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.