Something that is "past its prime" is no longer as strong as it used to be.
You can use "past its prime" to talk about people who aren't as healthy and strong as they were in the past. You can also say that a country or city is "past its prime" when it is used to be economically strong but is now poor and run-down:
People say that Detroit is long past its prime, but some still hold out hope that the city will revive itself.
If something is still young, strong, and healthy, you say that it is "in its prime":
In his prime, he was one of the strongest players in the league.