Age 21: You legally can now buy alcohol, handguns, lottery tickets and gamble in casinos or online.
Age 18: Legally you are now an adult, with all the responsibilities of adulthood and most of the perks. You legally can now vote, be in the military, sign contracts without parental consent and buy cigarettes, a rifle or a shotgun. You are are also old enough that having sex generally can be considered consensual. Your criminal record becomes permanent.
Age 17-15: Legally you are a minor and cannot be held to contracts. (Good luck buying a car on your own from a dealer.) Having sex is generally considered non-consensual depending on age difference. You could be tried as an adult for some crimes. Crimes committed as a juvenile are sealed. Incarceration for crimes committed as a juvenile ends at 18 years of age.
Age 14-11: Legally you are a minor for just about everything. Good luck getting into an R-rated movie without your parent attending.
Age 10 and under: Legally you are a minor, and generally not considered mature enough to babysit your younger siblings. Red Cross requires you to be 11 years old to take their babysitting course. Also generally, you are not liable for your actions. Hence the age restrictions on the high diving board at the public pool.
Notable exceptions for under age 18: A female at any age can get the pill or an abortion without a parent’s consent or knowledge in many states. Apparently at any age a minor is mature enough to decide what gender to be and obtain life altering treatment.