As regards whether young people are more likely to have accidents, surely the opinion of experts should carry some weight. People who do this sort of analysis for a living?
Look at the way insurance premiums vary with the age of the driver. You can even do this yourself with an online quote. Vary nothing except the driver's age, and see how it alters the cost.
So osprey is right, younger people are more likely to have accidents.
As to simply altering the age, chazza has a good point: you'd just move the risk to 4 years later.
What about letting youngsters learn to drive even earlier, at 16 say? But, prohibit them from driving unaccompanied by an adult until they are older.