The best years of one's life, when one is at the peak of one's powers, as in
She was in the prime of life when she began to lose her sight. The related phrase
in one's prime can be applied to objects as well as persons. For example,
The roses were in their prime when you last saw them. In both idioms
prime means "first in quality or character." [Early 1700s] Also see
past one's prime.