Usually, if they're old enough to crow, they're old enough to make chicks. Crowing is an indication that his little man hormones have kicked in and he should be fertile.
If he's mounting the hens, he's old enough to breed. Depending on the breed, the roo can be considered sexually mature by 4-6 months of age. Some breeds mature younger, some older.
Uh oh! That's scary ^ because I just found out today that my Sebright x Serama cross whom I thought was a pullet is actually a cockerel, unless I have a crowing hen lol, and I don't want any more mutts right now so looks like I'm going to have to keep him away from my Serama ladies.