You first must be comfortable in front of a crowd of complete strangers, ready with the right theme songs, and know how your tricks/props work.
I'd say if you ever placed less than first in any kind of talent competition, you're not ready.
I completely agree that a 13 year old is not ready to jump into the world of professional performances(especially walk around and restaurant style work), but some of the stuff you say here isn't relevant.
"Ready with the right theme songs". What? If he's doing walk around, you expect him to bring around a cd player and do silent acts at each table?
"Know how your tricks/props work". Knowing how it works proves nothing, knowing how to use them effectively in a good interactive routine, is what matters.
"... if you ever placed less than first in any kind of talent competition..." I've never been in a talent competiton, and I don't mean to sound conceided, but are you saying that I'm unqualified because of it? Entering a competiton does not have any bearing on your ability or professionalism. Honestly if I think a professional magician enters a local variety show, it would in a sense lower himself to a form of 'novelty act', and not the image you would strive for. If this is a magic competition, then it's different, but a talent show is nothing big to have on your resume.
Matthew, I remember about a year ago you came in here saying you were going to be doing kids shows and you were ready for it. Looking back, do you still think you were ready? Do you think that if you waited 6 months, you would have been more prepared? Now look at yourself again, think about how much better you'll be in 6 months from now, if you do some real research into the art of table hopping and get some experience by volunteering. To get more experience in closeup situations, offer to go to retirement homes/childrens hospitals and do some closeup for the people there. In this work, you will realize some of your flaws and improve on them before you try to really get work professionally.
Realisticly, if a business has the option of hiring a 16 year old magician who claims to have 2 years experience, or a 13 year old magician with 2 years experience, who will they pick? They're going to pick the older guy, because he's going to have more life experience and maturity. So think to yourself, if you wait a few years before trying to get work, they can have the option of a 16 year old with 2 years experience, or yourself a 16 year old with 5 years experience.
No matter how much you THINK you know, there is always going to be so much more that you don't. So why not try and learn? From simply reading books on magic history, you can get ideas you had never even considered before. Those who ignore their history are doomed to repeat it. If you don't research, you could spend 3 months trying to develop an idea that had been created 300 years ago, but you just never took the time to find it.
In other words, spend less time trying to get a gig with a big corporation, and more time doing your school work. Junior High is lame enough as it is, once you get past it then you'll have the knowledge, experience, and maturity you need to even consider starting to try and get work.
Just a couple refrences here for you to consider:
Kozmo's Live At The Jailhouse series, fantastic dvd set all about restaurant magic.
Peter Lansing's Making A Living Entertaining In Restaurants, great info from Paddy himself.
The Illustrated History of Magic, and Hiding The Elephant. Magic history books that I have a feeling you haven't read yet(just from your age), these will benefit you greatly and open many doors for you and your art.