Oh wow. Honestly, as soon as he starts shooting semen. That depends therefore on the individual because I have heard of some boys at twelve who already masturbate with the ability to ejaculate. Some supposedly can do this even earlier at maybe eleven or even ten. OF course, when boys brag about this, it may be a simple case of bragging. Some boys masturbate or at least play with themselves much earlier though ejaculation needn't take place.
If a boy is able to insert his penis inside a girl's vagina, then it would indicate at least the beginnings of the ability. SInce it only takes one sperm to impregnate a female, it would be best to assume that one encounter could indeed lead to a pregnancy regardless of the boy's age.
Do yourself a favor, do not play with a young boy this way. That boy would easily play basketball with the guys in the park rather than be responsible for a pregnant girl. Some would want to be but at that age simply do not know how. Best to stay clear.
Erick
