They've don't that for tobacco, alcohol, handguns, possibly soon to be for semi-auto rifles. Kids can stay on parents insurance longer. Driving restrictions have essentially raised the age until one can have full privileges. I'm sure there are others I'm missing too.
We might as well raise the legal age for everything to 21 at this point. Although my personal belief is that we're really just kicking the can - the kids don't learn responsibility until they have to and we're pushing that off later and later.