Depending on your family and background, coming of age could be marked by things like a quinceañera at 15 or sweet sixteen party. The next big birthday tends to be 18, when you can officially call yourself an independent adult, then comes 21 when you can finally order a stiff drink. Turning 20, however, normally gets you a cake with as many candles — and not much else. That is, unless you're Bella Thorne.