Am I too young to know my sexuality? I think I'm bisexual but my mom thinks I'm too young to know for sure and that I won't end up being bi when I'm older.

There's no right or wrong age to know your sexuality. Some discover it earlier, some need more time. But you're never to young to know - in fact, no one tells straight kids they're too young to know, it's a stereotype that people only apply to non-straight people. Our orientation is nothing more than what we feel we are, so if you feel you're bi, that's who you are, whatever your age is. If later you'll feel different, you can identify with something different, but you should be free to identify with whatever feels like you now. And after all, there's no reason why you should feel different later, you're perfectly capable of understanding who you are now. There's no such things as too young.