im guessing you mean male dog breeding from he's.....
I think its supposed to be a couple of years old to ensure he has matured fully both physically and sexually, and then you also get a good idea of temperament etc so whether he is a nice enough dog to breed from
Not sure of an exact age but I think its 2-3years minimum.... I may be corrected here!
I certainly wouldn't breed from any male under three years, if he was unproven or didn't have acceptable scores/results for the health tests associated with the breed.
Also needs to be sound in mind and body.
I think different breeds may have different guidelines.
And I don't intend to breed for the forseeable anyway