Sadly, we generalise too much. There are good, average and awful drivers in all age ranges. Some youngsters do draw attention to themselves by making the "modifications" to their cars that they do, and granted, a few will take risks that the rest of us would never dream of, but the same applies to Mr Rep in his company Audi/BMW at times. At the other end of the age range, we do see the more mature person who causes problems by dithering and bumbling along completely lost in their own little world. Conversely, a little while ago I conducted a driver assessment with a chap who'd recently turned 80. He told me that the week before he'd run up Snowdon for the last time as he decided to give up running when he turned 80! As for his driving (a 5.0-litre, supercharged Range Rover Sport), he'd put many to shame, and was VERY safe. I remember my son when he turned 17. He said that one of his ambitions was to be a better driver than me. I told him to get on with it, and encouraged any training that he could get. Whilst he wouldn't be able to have the Police training that I had (but believe me, there are some who pass those courses, but don't maintain standards and I personally wouldn't give them a dog licence (for those who remember those!), let alone a driving licence), he has taken various tests, and he is one of the few people that I feel totally comfortable as a passenger with (I'm an awful passenger). And that isn't commenting upon my own driving, it's purely that I'm a bad passenger!
The driving licence renewal post 70 is a farce really, as it's pretty much a self-assessment as to health rather than having any practical driving assessment. However, as I said before, I would not single out any age group, but would have assessments for everyone.
As for reactions, in the driving world, studies (Johannson?) have shown that the average reaction times are between one and two seconds. What might surprise you is that between the ages of 17 and 25 the average reaction time is nearer two seconds (inexperience), 25-55, it comes down to one second, and then 55 and over, back up to two seconds (because we start slowing down). Forget the Highway Code nonsense of 0.7 seconds, as that's done in laboratory conditions anticipating a change of a light colour with nothing else to concentrate on.