A male can impregnate a woman as soon as he begins manufacturing sperm. So, 9, 10, 11. And a female can be impregnated as soon as eggs start rolling out of the warehouse (fun fact if you didn’t know; women are born with every egg they’ll ever produce, already pre loaded in their ovaries) So technically speaking, neither age is relevant.
Once puberty hits both genders, babies can be made. It becomes a question of morality at that point. But to answer your question specifically, anywhere between 9 years old and whenever he dies (100 years, 120 years. Although, a 120 year old impregnating a 29 year old would probably kill him)