I'm not 100% sure this is right, but it might even have to do with hormones, as you learn certain things, you may want to take them less seriously, as a way of kind of escaping the world. Being immature about something is kind of like turning a serious subject into a comedic one. Certain immaturity can be bad (in other words, doing certain things b/c you think its funny when it could really be hurtful to others), but otherwise, have fun while you're still young. You're only young twice (
), so just keep on keeping on. Have fun, but just be aware of when you're being immature. I know that around teachers and adults (with the exception of my parents) I'm mature. But around my friends and my parents I act really immature, and when my friends act immature, I join in on the fun, although sometimes I know when they're going too far, and I get them to stop being idiots, but still, immaturity is basically part of life, part of growing up. You have to deal with it, just don't act like an idiot around your teachers, and don't be an idiot like my friend and be immature in front of a cop (by being immature in front of a cop, I mean giving a cop the finger when he's staring right at you). Being immature is perfectly normal!
And not being able to control it is also perfectly normal!