Until recently we had 3 gilts running with one boar. All were approx 4-5 months old at the time. The boar we are keeping for breeding, the gilts will go to slaughter. One of the gilts recently came into season and the boar was sexually active! As pointed out in a previous thread, in the event that she is pregnant the gilt will go to slaughter well before the due date, but we didn't like the idea of potentially sending a pregnant gilt to slaughter.
I have now separated them and he is now running with an older gilt that we want him to cover as soon as he is able!
So, to get back to your original point, I'd certainly go with Roberts suggestion and we will not be mixing gilts and boars again ... unless of course that is the intent.
Hope this helps.
Pete