sorry I felt this was necessary to add on, at 18 you can die for your country... at that point you should have all your rights. I could see a legit argument made for 25 but 21 is so arbitrary.
I've always hated how, in my country, you can drink, vote and join the armed forces at 18, but yet if you go to the movies, at 16, you're an adult, paying adult prices - but can't see an R rated movie! WTF!?
So interesting note. If you grew up watching Boy Meets World starring Ben Savage, there is in the works a sequel series, called Girl meets World, that focuses on Corey's daughter. They are bringing back Ben and Danielle to play Corey and Topanga.