And no, there's nothing "scary" about requiring people to undergo periodic road testing after they reach a certain age. There isn't a single human body function that doesn't decline over time, and you'd probably be shocked at just how poor your typical retiree functions by any measure (vision, motor skills, mental sharpness, etc.) compared to people in the 18-50 age group.
That 18 year old may have better reflexes but he has a whole lot of hormones and almost no real-road experience. How many elderlies do you see wiping out at 100+ on a surburban street?