High Risk Age Groups: People between the ages of 15 and 24 and over 75 are the groups most likely affected by car accidents
You say teenagers will most likely become a better driver, but that does not take away that year in year out they are the most likely to be involved in car accidents. Older drivers are seen as safe drivers, however because of their age are more likely to die in a car accident. The way forward would surely be to ask drivers who have been involved in a accident that is deemed their fault, to have some sort of retest/training paid by them before they get back behind the wheel. Better to address the poor driving standards, whatever the age, then to pick out one age group.
Problem is then you are only stopping people driving after they've had an accident, rather than trying to do it preemptively through stricter controls.