I'd say 30 was not too far beyond my peak both physically and mentally (in terms of sharpness, not knowledge). At that point, you don't recover as fast as your early-mid 20s but everything still works pretty well.
Mid-30s was roughly when I started getting the 2-3 day hangovers, pulling up sore after moving apartment, feeling my back if I sat all day, that sort of stuff. And athletic performance is less sharp, reflexes a little slower. Mental performance too less built for endurance sessions like cramming 24 hours for an exam, but also things like 3-min chess games I started getting worse after maybe 30 mins and needing a break where before I could go for hours at basically the same level.
Now early 40s, it's just kept degrading slowly. Can still do basically everything I could do at 20 but its all a bit slower, less explosive, bit more creaky, needs more recovery. Both physically and mentally. I had a minor tweak in my neck which in my 20s would be fixed the next day that took months to get over. And it recurs. My hearing has also started to go when there are competing sounds.
Everything just gradually gets a little worse year by year. I kinda like that its slow enough you barely notice it - only when you remember what you could do once.
