I think the current practice in most countries works okay. Anyone beyond 18 should be given a temporary license with which they can practice. They should then be rated purely based on their driving skills. In a perfect world this will work just fine.

But problem is anyone will tend to find a way to cheat the system after doing mistakes. With 18-20 year olds this might be even more so, because they would not be earning full time! It is up to system (& infrastructure) to stay up to date and catch such folks red-handed.

The only deterrent to such obnoxious behavior will be high insurance cost for law breakers. I think 18 year onward will be a good time. Anything further will mean younger kids will get into driving illegally i.e. without license & they will end up sitting behind the wheel without any practice!