Both women and men are at their physical peak in the early twenties, it's all downhill after that.
Scientists generally do their best work before their 30th birthday. Over 30 is past your intellectual peak if hard thinking is your game.
When I was a teenager, middle-age began at 30. (It's not called "the big 3-O" for nothing.) 50 was past it and 60 was old.
Having said that, different people age differently. I met an 80-year-old man in a ski resort who was 5 years older than my father at the time, but seemed 15 years younger.
I saw a photo of my high-school class 30 year reunion, I thought nearly everyone in it looked 15 years older than me. This wasn't just vanity self-delusion, there were a couple of other people who looked the age I see in the mirror.