At least old enough to drink alcohol. So 20 right? What happens when girls enter war? They get raped. If Sarada gets captured, her sharingan is not the only thing that will be stolen. Her innocence. And by that, I mean her virginity. How will boruto marry used goods.
15 is a decent age.
You would die for a village that wouldn’t even let you drink alcohol? Too young to smoke, but old enough to get a Kunai in the face?
Nah, at least 20.
That's what happened to Karin was one of the saddest things in the series