My dd will be 5 in May, and is still wearing nappies at night time. The nappy is always sodden in the morning, and she's definitely not ready to be dry yet. I'm not concerned about this at the moment as both dh and I were late being dry at night and it does run in families; I also know that there's a hormone that kicks in to suppress urination overnight and until that happens she will struggle to be dry.
Dh was wondering, though, when it starts to be something to be concerned about and wanted me to ask the "fount of all knowledge" - you lot! My HV said once that I ought to take her to the GP for referral to a continence specialist if she was still wet at night by the age of 5, but that seems a bit early to be worrying to me.