Dizzy, you can be both mature and immature at times. Sometimes the immaturity is a little annoying, but everyone gets like that at times. You're better than a LOT of the members here.
As for myself? Well, it really depends. Sometimes immature people annoy me, but sometimes they don't. See, I try to be myself offline AND online. So what you see me typing here is how I would really say it if I were telling it to your face. As I'm sure you've seen at least once during your time at PC, sometimes I get... hyper. XD I can get pretty immature at times myself, especially when I'm at ease and around friends, but I'm also mature at times (well, I like to think so anyway ;p). So it would be unfair to get annoyed by all people immature. I find them quite amusing most of the time, actually.