At 21, I can drink alcohol and gamble in the US legally now. I can apply to adopt a child. Train to be an airline pilot. Supervise a learner driver by law. If I commit a crime, I will be tried in a court of law as an adult around the world. If I married in America, it would no longer be law to provide my birth certificate or parental consent. I can now buy knives legally, (oh, and an axe if I like.)
Exciting ey? Some of the celebrations have already occurred this weekend, and there's more still to come, if my immune system doesn't collapse by then.