He can become sexullay mature at around 5 months, but it is more a case of mechanics at that age - ie is he tall enough to mount your gilt/sow. Typically I would let him go through to 8-10 months, then choose a small sow (so one of them knows what to do!). If your choice of female is limited, then you may have to just wait until he is big enough or buy him a stool !
From about a year old you could expect him to be working normally, and whilst he will continue to grow, he'll be big enough to cope with most sows by then.