My son is 6 1/2 and still wets the bed. He has been potty trained for many years now and has no accidents at all during the day - ever. I talked to his doctor about it in length and he said to only start worrying about it if he's still doing it by the time he's 15. That blew me away, but then he explained. He said there is a hormone in your body that "switches on" when you're sleeping that slows down the production of urine. It's a type of safety mechanism in your body. For instance, if you're in the desert and you're dehydrated, your body will kick in that hormone to stop producing urine because it knows you need to retain all the water it can. The same thing happens when you sleep. It doesn't stop producing altogether, but it slows it down quite a bit. My son isn't producing that hormone yet; therefore, he continues to produce the same amount of urine when he's sleeping as he does when he is awake. MANY kids have this problem.
The second part of the problem is that he sleeps so heavy that the urge to pee doesn't wake him up, which is another problem many kids have. My doctor said your body can be trained to wake up when the urge is there. He told me about a product called the SleepDry Alarm. It's a little alarm that attaches to his shirt, with a wire that attaches to his underwear in the front. When he's sleeping and starts to pee the moisture hits the wire and sounds the alarm. The moment the alarm goes off it triggers him to stop peeing and wakes him up. I worried it wouldn't wake him up because he's a heavy sleeper, but it does wake him up immediately and it instantly stops him from continuing to pee. He detaches the wire, gets up, goes to pee, changes his underwear (which only has a tiny wet spot) I help him re-attach it and he goes back to bed. After a while the idea is that his body will learn to recognize the urge, even without the alarm.
We've been using it for a week and a half and it's working well, as far as waking him up before the bed is soaked, or even before his underwear are soaked. Every child is different so it could take one child three weeks or another child three months. It is a learned behavior, so you never know how long it could take.
So far, I'm very happy because we haven't used one single pull-up and haven't had one single mess on the bed. He's very pleased with it, too.
You can Google SleepDry Alarm and read all kinds of feedback about it from customers. Hope that helps.