We're in the same boat. My DS2 turned 8 in Dec and still is not dry at night. I have heard that there needs to be a hormone present, like the pp said, but it takes longer to be produced in some kids then others. We're just going to wait it out. We do goodnights and a wool soaker or shorts over top. And invest in those crib sized waterproof pads. We don't really make it an issue, he can't help it. He will strip his bed when his diaper leaks, and remake his bed, he feels bad, but it's just not something he can control.
We have bought the big pack of depends at Costco for him before, they seem to do better for a larger bladder. Just fold the waist down. They have good coupons for those!
~Rebekah~Mother of 9, 6 of whom I have the honor of raising, 3 with Jesus..