I guantee that you will run into crabby people. If that happens, just say "OK, see you later" and leave. It whappens all the time. Because you're a kid it probably will not happen as often to you as to me as an adult, people don't want to hurt a kid's feelings.
Actually Paddy it would probably be the other way, as people don't want to disrespect someone thats older, especially if the performer is quite a bit older than them, but they could care less about disrespecting someone thats younger. I have noticed that people HATE to walk off from my boss performing at the magic shop I work in without buying anything because they feel like they're being rude to him, and I have even seen a few times people check the prices and buy the cheapest thing because they feel uncomfortable not buying something from him after watching him demonstrate.
But they don't care at all to walk off from me because I'm only 20, so generally they're typically older than me by quite a bit. And I think it was even worse was when I was younger too. I think younger magicians will get heckled a lot more often too because people think a "kid" can't possibly fool them, which isn't true at all if the "kid" has been doing magic for a long time and has a lot of experience performing.
Cstaskel I suggest trying to work in a magic shop first though if you can, or try to find some way to get some hard performing experience before hiring yourself out. I have been demonstrating in a magic shop 5 years now and only in the past year to two and a half years roughly have I felt like I am capable of performing well for close-up stuff or table hopping.
BUT, if you can't find anything to perform like in a magic shop, then just perform constantly at every opportunity for everyone you can literally, and after doing that for at least a few months consider hiring yourself out.
At some point you do have to start though if you intend to lol. I'm going to be table hopping myself shortly probably.