Aside from what everyone else has pointed out about how you'll probably keep growing, (it's not just height, your hips may grow or you may gain or even lose weight) I don't think you'll be able to get professional mermaid gigs until you're like, 16 at the earliest. Child labor laws and all that. There are exceptions for child actors and models and the like but I seriously doubt there's much demand for experienced child mermaids that you'll be able to get any gigs before you're 16.

Just do mermaiding for fun for now. If you're really serious about doing it professionally, work on skills that will help you do it in the future. Practice swimming as much as you can, and see if you can take synchronized swimming lessons. Once you're able to start making money, save it. A silicone tail is not a necessity, even for an aspiring mermaid (there are plenty of lovely fabric and neoprene tails) so it's really not something you should be asking random people to give you money for. I know it's hard to be patient at your age, and it can be pretty annoying when everyone is giving you the same advice that you don't want to hear, but there's usually a good reason so many people are giving you that advice. Reaching a dream takes a lot of work and patience, and becoming a professional mermaid is no exception. Professional mermaiding is more than just putting on a tail and swimming around looking pretty, otherwise I'd be doing it, ha ha.