That varies a great deal, but a general rule of thumb for British breeds is they are fertile by about 11 months and should easily breed and settle cows by 14 months. Continentals are pretty much the same, maybe slightly later and Brahman bulls about 18 months, but these are only a rough guide.
I am currently using a 14 month old hereford bull as a cleanup bull. It is important though not to overwork the boy. don't give him more heifers/cows than he is old in months to breed in a 60 day cycle.