Not to be Debbie Downer, but if you have no prior experience with dance or tumbling by the time you are 16 then more than likely competitive cheer just isn't for you. I'm not saying someone can push themselves like crazy, just that most don't. It's significantly easier to learn necessary advanced tumbling skills at a younger age.
I know I will get blasted with "but so and so on team x did it" yes, so and so and whatshername may have done it but in general, the girl who tries to make the squad at 17 when she's never cheered or tumbled before will more than likely leave heartbroken and not on the squad. If you want to be a successful cheerleader, as with anything else, you SHOULD start early.