Driving is, necessarily, a state thing...in some (Midwestern) states, the roads are flat and straight for miles and miles, as is the terrain around them. You can see cars or obstacles miles off, and in these areas licenses are issued to younger folk. Other areas the driving the trickier. Regardless, far too many 16-21-year-olds have real need to drive for it to be reasonable to raise that bar higher. Voting and drinking are national, and thus are very different from driving. There aren't really any geographic reasons why one area should be able to drink at younger ages, nor is it a necessity in anyone's life. Point is, discussions of ages and rights probably should be focused on voting, drinking, and other things, but not driving.
Now, I personally would like to see the drinking age be lowered to 18 (obviously), but I'd be content with the decision being given back to the states. There was a time in American history when prohibition was a national movement...that day is past. Some states don't allow children (or anyone) near fireworks, but some states allow all kinds for everyone. I haven't ever heard of any terrible problems with that system.