If you are still reading these posts....it really depends on the person, what you like about them, and whether or not you can stomach the annoyance. I understand what you are saying, how a person display's themseves may speak color's about who that person is, and how great of a connection they have with yourself. Do you want to make this work, only to find that you are two very different people in the long run.
I guess thats all a question of length of time you have been in the relationship, and if his immaturity is a reflection of your own personality. Some people are just not in the correct place in life and frame of mind for a relationship with each other. That's a cold harsh fact of a relationship that when taken into the long term can only lead into heart break. Typically partners will then desire another person to complete what they feel they are missing out from, and that is something you do not want to do or have done to yourself. So you have to ask yourself, do you wish to be with this person and grow with them, or move down a seperate path away from who they are now? If you are happy with meeting him half way, than my suggestion is to give it a shot and perhaps let loose when you can. Life is short but also long, and a few years can mean the world.
I personally matured in many ways due to oversea's time (military), but have always had a joking-goofy way about me. That is not to say that I am not insightful in area's in which I am interested in...but being immature in some way's is a release for me from the experiance I have had...mainly oversea's (btw, I'm 26 going on 27...takes some time to mature). This does not effect my morality or how I typically display myself, but in comfortable settings I sometimes wish to act on repressed excitment; such as with a loved one, close friend or family member. I had a girlfriend when I was 21 by the name of Tina. We ended up having a long term relationship because she too did her schooling away from home in Nelson B.C, and she and I are from a different province. She was a few months older than me, but to her understanding she felt "she could not have fun because I was too busy joking around". Our relationship ended in heartache when she cheated on me. But you want to know the part that really bothered me (other than the lack of understanding at the time)? The fact that I told myself on many occasions that I should break up with her, but didn't.
If you are having these same thoughts and feelings, it will probably be better for both of you if you communicate, find a hard hitting reason to break up (difference in desires for kid's, marriage, travel, etc), and have a break up with a mature reasoning. It will then be better for both of you, and either party can move on with hopes that one day you will both be on the same level.
That's my advice, I hope it helps.