Julia Child didn't become Julia Child until she was in her 60s. If you're in fairly okay health you still have a few decades to do what it is that you want to do.
Most of them are in their 40s, 50s, and 60s [0]. For many of them, it's only now that they're reaching the peak of their professional careers. These people weren't major figures at the outset, unlike other actors.
[0] https://www.google.com/search?q=better+call+saul+cast+age&hl...