It's not an age realated issue. What I did when my 11 year old potty trained and am now doing for my 4 year old twins (GB) is they wear pull-ups to bed until they go a solid week with a dry pull-up in the morning when it's time to get up and go potty. Once my oldest hit that week (she was 3.5 years old at the time) we celebrated and put her in underwear that night. She has never had a night time accident since.
My twins are still working on it. They are fine during the day, but my female twin is unable to make through naptime or bedtime without and accident and my son can make it through naptime, but not bedtime... It's individual and has a lot to do with recognizing the body's signs that they need to go.
I think putting them into underwear before they are ready sets them up for disappointment and discouragement and causes the whole process to take a lot longer. Just be consisitent and make sure to take you son to the bathroom first thing when he gets up and check him before you go to bed at night to see if he has already wet himself. If you are getting him up to change him when he is wet, he may learn to recognize his body's needs faster. Just make it okay for him to get up and go in the middle of the night if he needs. Leave a path of nightlights from his room to the bathroom...Stuff like that.
Good Luck ;-)