Well, i'm 14, going on 15 on the 19th!
I've been seriously keeping fish for about 5 years. I've had animals all my life (mainly reptiles), but after my many transistions to various types of animals I kind of stuck with fish. I still do keep a few reptiles here and there though.
What I don't like about this hobby is when your younger people don't like listening to you, they'll take a 40 year olds word over yours even if it's highly strange and messed up.
Being almost fifteen, does is inhibit your ability to care for an animal that most people can keep alive for 2 days? No, the only problem I have with caring for my animals is that i'm not able to go and buy a new canister filter out of the blue if my old one breaks or something. I have to save and save and save before I can get anything. I'm about to get a job at an lfs, so it might change a little.
I think fish keeping when your younger seriously helps you out in school, helps with stress, etc. Heck, I was the only kid in my class that knew about nitrites, nitrates, salinity, the Ph scale, and the like. When all others were basically just starting the test on the Ph scale, I was already done. It's helped me out with school a lot since i've been keeping fish. You don't really know it but you learn some of the hardest subjects in school by just keeping a tank filled with water and a few handful of fish species.
-flamingonhot