Objectively? Around 35.
35 really. But people ain't gonna like that. And I say that as someone in his early 30s.
Us old heads
would like to think it's 40 but it's 30. 35 at most.
Yea.
The basketball/hooping example mentioned earlier was spot on.
Into your early 30s you can still have a very active physically intensive hobby like bball 4-5 days out the week and feel perfectly fine... but trust, sometime shortly after you turn 35 (could be 36 or 37) something just happens and all of a sudden the body just ain't recovering like it used to; you accumulating all these minor injuries (or a major injury) that you never had to deal with before; and now every morning after feels like you just did a heavy leg day. You really gotta double down on the preventive maintenance; icing, knee/elbow sleeves, stretching, etc.
For those single/dating, that mid-30s is also when you may notice that the younger females (early 20s) that you could still scoop up at 30, now find you either too old or they look at you more-so as a provider. Basically the "old guy in the club" thing. Women in their late 20s will probably ask why you aren't married/don't have kids yet.
Work-wise, you're still relatively young but some argue that you should have your plan of action set/established by 35 (whether that's the corporate 9-5 route, you're own business, or whatever); granted that's obviously not true for everyone but you're likely to get shunned if you still got nothin going for you by mid-30s.