A car is close to a necessity in many non city areas. I think it should be treated essentially as it is, due to the social and economic negative effects it will have if certain people can't get their license, and the effect it would have on their children (income a major issue).
I think raising the costs of getting one would have the most effect on the poor, which is never a good thing.
I don't really care about the whole 16/18 thing, but I would strongly oppose increasing restrictions upon actual adults with livelihoods. It's one thing if someone repeatedly shows they can't handle it (ie. multiple dui's), but we already do that for the most part. Though driving restrictions (ie. no highways) I may be open to if done properly and conservatively.