sport
Grew up playing lots and lots and lots of football (soccer), up to to about 14 when i effectively stopped, I would occasionally play football, tennis or swim. I love team sports but if we're playing basketball or volleyball im making your team worse lol.
[I think being in good health is super important sleep, eat well, exercise, etc]
Nowadays I try to stay in shape by doing cardio (running mostly), and pay attention to lung capacity, resting heart rate and coordination. Lifting weights and body weight exercises. Occasional stretching or core exercises (can touch my toes and handstand). Also a little tai chi and climbing.
Whilst most physical activity hobbies are super fun, I think from a health point of view most young people don't actually need to exercise that much, you're not at immediate risk of anything. I think the amount of exercises you need to do increases as you age. However getting into hobbies and particularly physical exercises is exponentially harder when you already (respectfully) suck. Unless you've done something similar before, you probably will suck (lol).
On the flip side, at the beginning, every additional hour will lead to a bigger improvement than the next, so after accumulating 1h, 10h then 100 hours you're probably on the other side and can begin enjoying. At that point the skills stick with you, so you could jump back in when you have time with less difficulties.
So yeah... the best time to start is probably now, don't injure yourself, and its okay if you're not Messi at first.
If you have any sports you currently do that you think I must try out, let me know?