When do you become an adult? Kids think they are as smart as adults, and middle-aged people say they still don't "feel" adult. Maturity is a gradual process, a spectrum in which the amount of wisdom is hard to pin down. The question of when someone is mature "enough" for certain responsibilities has legal ramifications, so we set an age, whether it is applicable to everyone or not.
How does a culture set the age of consent? It depends on consent for what, then it depends on the culture, and on the real goal of setting a limit. In America, we legally allow 16-year-olds drive but not drink. In some European countries, one can drink at a much younger age than one can get a driver's license. It makes sense when you realize that Europeans can get around without driving, while in America, cross-country mass transit means buying an airline ticket. Simon Whistler of Today I Found Out tackles the question from several angles.