1. I always felt that I had too little time to get used to one "stage" of things before moving onto the next one - like I had just learned the rules and then I had to learn a whole new set of them all over again. It also seemed like there was always too many more responsibilities and far too few freedoms that I actually cared about.
2. I never want to FEEL like an old person. It's really hard to conceive for me, mostly because I don't even feel like a twenty-something, or even a teenager, most times. I'd also really, really like to avoid all the medical complications that come with aging. I've set foot in a hospital far too many times in my life already, I'd rather not have to again.
3. That's... hard. Do you mean in the way that it's usually meant, or what I think should qualify as maturity? If the former, "grow up" usually means to give up your dreams, settle down and do what everyone else does because it's socially appropriate. If the latter, I'd say that means doing what you have to do in order to get to where you want to go, thinking things through before you act, doing your best to maximize good things and minimize bad things you do when it comes to dealing with other people, and consciously trying to make the best possible mark on the world in the time you have in it.