I'd agree with others that 8 to 10 months (as long as he's well-grown and healthy) is a good age. However, for the first mating, you need to be there to supervise. Two young virgins can fumble about but not achieve anything - and they may injure themselves in the process.
Make sure they are on level ground (my paddocks are churned up like the surface of the moon and frozen solid at the moment) so there's no chance of slipping. Try and match the height of both parties, too.
Now you may not want to do this, but you might have to - quite literally - lend a hand. A young boar often has all the right intentions, but no real sense of direction, and pots the brown instead of the pink, so to speak. Helping him get the job done will help both him and the gilt/sow.