Start when you can. I started at age 18, I'm 19 years old. I don't know if I would have liked to start earlier, although I've been curious about jiu jitsu since I was 16. I think that, if I started in my early teens or as a child, I could have ended up quitting. I loved soccer as a kid, but left due to a bunch of dumb reasons and lack of willpower. By starting now, I think I can "tough it out" and I'm not terrified of pain and setbacks anymore. As dad says to me, "what has to be, already is". Do what you can.