It's not so much - how old he is - but more important - how much he weighs.
A normal 7 month old 700# bull calf CAN settle a female - very rare - but can happen. Even if HE is developed enough to produce live sperm, rarely is a female small enough for him to penetrate is actually producing a viable egg.
If he weighs 1100-1200# on his birthdate, and you wanted to breed cows that early, he would probably be able to settle 5-10. Actually, the recommendation is 1 cow per month old. So 12 months - 12 cows. 15 months old - 15 cows.