The law is a blunt instrument. It will take some semi-arbitrary generalization,I guess it's more a matter of what age one is considered "mature." There might be some 15-year-olds who are more mature and responsible than those who are in their 40s or 50s (or even older).
& make it strict law. Let's say they pick 18 years of age....as you point out,
this will be unfair to some, & dangerous with others. But it should yield a
better result than the alternatives.