Laws preventing underage people from doing certain things create a bright line. Some kids are probably capable of driving ar 12, while I know plenty of grown-ups who drive like 5 year-olds.
The thing is, the fact that a few 14 year-olds can drive is outweighed by the expense of allowing all of them to apply for a driver's license.
Age of consent laws aren't perfect. Without them 14 year-olds could drink, smoke, drive, strip and perform in pornography. Not a positive result, IMO.
Fair enough.