You must wait until they're a year old to let them breed - though I don't recommend it, as there are already so many gliders out there looking for a loving home. Regardless, they will become fertile soon enough. It is probably a couple months away, but you never know... if they are allowed to breed before 1 year it is likely the joeys will not survive. We had it happen... our female was old enough but our male was 10 months or so. We accidentally let them breed, not realizing how easy it is for that to happen. The dad showed aggression towards the babies, the mother rejected them, and one died of hypothermia because the parents just couldnt take care of them. We saved the other by pure luck. If they are in the cage together, they will mate. It's not very predictable and can happen any time. They do go into heat, but there is no way you could control whether or not she gets pregnant. You have to either get him fixed or keep them separate until they're 1 year - in which case, each would need a same-sex cage mate so they don't get depressed, and then there's the issue of those same-sex pairs becoming bonded, which would make splitting them up later on immoral... it's tricky. Be careful, gliders simply shouldn't have babies before 1 year old. Also, I hope your female isn't actually that young, because they should not be separated from their parents until 8 weeks minimum. If you were sold a 3-4 week old glider, that breeder needs to be shut down because that is extremely dangerous. Good luck.