Let's say 30 is too young. That's when you're likely to finish your PhD and be relatively inexperienced. I was probably smarter at 30 than I am now, but I didn't know a lot. Let's say 60 is too old for the "peak." You can still do good work and you're going to know a heck of a lot if you've been a scholar your whole life, but the ideas are not likely to be as fresh and original. You're likely to be repeating yourself a bit.
So the peak years are probably between 35-55. I personally feel I'm peaking now, but I don't actually know if I'll ever do anything as good as Apocryphal Lorca again. Even if I do a few more books on the level of my least accomplished book, I'll be happy.
The other theory is that it really doesn't matter how old you are. You could be Rimbaud or Sophocles. I'm still exploring ideas I had when I was 24. We're not tennis players who are old at 30.