Sweeping generalization.....
Young drivers, are on the whole, dangerous, they all think they are tasty drivers and the normal rules of the road don't apply to them.
Old drivers are, on the whole, blind and slow, therefore dangerous.
Which is more dangerous?, no idea, about the same to me.
I'd say 90% of drivers, regardless of age, are CRAP, the amount of people who have no idea of lane discapline, roundabouts, rights of way, indication and just general road sense is unbelieveable.
Compulsary retests for EVERYONE, every 5 or so years is the only way to go.