Both my older two boys are like this.
Aiden truly believes he is 7 (he is 5.5 now) and Nathan believes he is turning 5, maybe 6 on his next birthday (poor kid is only turning 4)
Aiden's play therapist told us that this is not just an "I wish I was older" thing, but that he truly truly believes that he is a 7 year old trapped in a 5 year-old's body.
She also mentioned the autonomy thing and since we give him more personal choice he is a LOT better - calmer, happier and more content with the frustrations that a 5 year old has to face
I think that you can also do a lot of good for these kids to acknowledge their feelings, allow them to feel them, to explore them and let them know that you love them regardless of age, ability and anything else.
When I say to my kids that it's time to stop playing computers (for example) and they moan about it, I feed their wishes: "I bet when you are an adult you will play on the computer all day!" Sometimes they just need to know that you UNDERSTAND them and their desires and they then understand that its just not possible and they they are able to switch off the PC (or whatever else it is) quite happily.
Since we started doing it like this it makes all the difference, simply because they feel that they are heard and understood.
