Make the test harder for everybody. Too many people on the road don't really know what they are doing.
I agree with this. The testing should be more difficult. I'd go one step further and say that everyone should have to periodically retake both the written and driving tests. I'm thinking the written test should have to be taken every time you renew your license, such as every five years. On top of that, once every other time (say once every 10 years), you should have to retake the driving test. Some people may ask about those that fail. I would ask if you really want those people on the road. It doesn't really make sense to take a test once (at least if you stay in the same state) and suddenly be granted a license for what could be several decades.
As for the age itself, I could see allowing for a permit at either 15 or 16, but not a full license until at least 18. I think for anyone, if you're going for your first driver's license, you should have to hold a permit for at least six months (maybe even a year).