My son and daughter are still in night time pull-ups. My son is 7, daughter 5 next week. It runs in the family I guess, my doctor is not concerned and at least I know of other kids older than mine who still have trouble during the night. I believe the pull-ups go up to size 6 (40-60lbs) and maybe even heavier.
But, I thought I'd mention the ideas my doctor gave to help my kids make it through the night, it has worked better and my daughter has had many nights with dry diapers and my son is now only going one time during the night.
* No drinks 2 hours before bed (this is one we
have a hard time with but we try to just give
sips).
* No carbonated and especially caffinated drinks
after lunch
* (Here's the one that surprised me): No candies
with red dye, this includes fruit snacks,
twizzlers, m&ms, etc.
* Of course have them go to the bathroom right
before they get into bed.
Good luck and remember to try to not make a big deal about it; I had a really hard and frustrating time with it at first, but the doctor explained that everyone has a different time to train at night. So, now, we just encourage them to try their best, keep these guidelines and celebrate when they do make it.