Yes, if you are too young, people will not take you seriously. You will not get a resturaunt job if you're too young. One, it's against the law, and the owner will look unproffessional.
Whether an audience takes you seriously or not depends on a couple of factors - one of those factors being the audience. A lot of adults know not to patronize, and won't unless they're drunk or a bit narrow minded. If what you show your audiences impresses them (and you don't need a fantastic performance to impress), then they will take you seriously; people realise that it takes a lot of courage to stand up and show something "different" to a bunch of strangers. Granted, you will not be received as an adult would, but you will still be taken seriously the majority of the time.
I don't believe it is against the law in England to get a job in a restaurant under the age of 16. However, 15 is usually the minimum, which is true. But as long as you don't harass the customers and drop shiny magic props in their dinner, then the manager won't look all that unprofessional.