Driving at 16 may have been the biggest one--that freedom that comes with wheels, even when you manage to wreck those wheels a couple times (and they don't belong to you--Sorry Dad).
Drinking at 21 was some sort of milestone, but most people don't pay any attention to that one. What other age matters? Voting? Okay, that 18th birthday--legally recognized adult and voting and all... But who at 18 really cares about that. R-rated movies at 17? Nope.