Very young and very old will warrant my attention until assurred of adequate performance.
That is your prerogative, but it has no clear basis in objective reality.
Youth isn't a problem. Lack of experience is a big problem. Young people generally have less experience. But by checking experience (and not age), you solve that problem without the need for age discrimination. If a pilot somehow has 15,000 hours of real experience at age 25, then the fact that he is 25 makes no difference at all.
Old age isn't a problem. Medical conditions that are more common with age are problems. But the medical exam rules those out. If you were pulling 70-year-old people at random from the street to work as pilots, then yes, there would be more medical problems among them than you'd find in a 30-year-old cohort. However, pilots are not pulled from the population at random. A vast body of regulation and testing ensures that all pilots meet certain physical qualifications for flying, so irrespective of age, a pilot who is certified medically fit is okay to fly.
I guess I was dreaming about my past experience of flying with age 60+ pilots and the proportion of them demonstrating performance limitations.
It might be biased or wishful thinking.
Remember also that marginal pilots deteriorate a lot faster than superior pilots. That's not a direct result of aging, it's just that aging makes mediocre pilots a lot more obvious.
[The doctor I know] is a firm believer in a fixed age and has told me the problems he faces trying to assess the mental conditon of old pilots during the time it takes for a medical, almost an impossible task.
The assessment is the same for all ages. If he has doubts about the mental condition of an older pilot, then he needs to defer or deny the medical as appropriate, if he has reason to believe that the pilot is unfit.
From what you are saying, this doctor takes for granted that older pilots require more evaluation, even though there's no evidence to support that.
A simple principle applies: Any exam that accurately assesses the aptitude of a pilot to fly can be applied to any age, because age itself is not relevant to aptitude. If the current exam fails to identify certain pilots as not fit to fly, then the exam needs to change. You do not administer a different exam for older pilots, because the objective requirements of flying do not change with age (maybe they change with aircraft, or type of pilot certification, or with other factors, but not with age).
Doctors can be biased, too, unfortunately.
I'm surprised (well, maybe not so surprised) that so many here dwell on the alleged deterioration in physical condition that comes with age, while ignoring the huge, enormous advantage that comes with experience. Not just total flying hours, but general experience. Older people make decisions more slowly, but their decisions are generally better. That's why so many positions of responsibility are held by older people. And flying an aircraft these days, at least when it comes to commercial air travel, is not sufficiently demanding physically to justify excessive worry about medical condition, whereas the value of experience continues to increase every day.
Look at the accident record. How many accidents were due to slow reflexes, and how many were due to poor judgment? Reflexes deteriorate with age, but judgment improves (not because of age itself, but because of the increased experience that usually comes with age).
Think back to that Colgan Air crash. The pilots were young indeed, and perhaps they had cat-like reflexes in consequence; but they made all the wrong decisions, and so, instead of rapidly getting out of trouble, they rapidly got deeper into it. Would a 65-year-old pilot with thousands of hours of experience in similar situations have made the same wrong decisions? Would he have even allowed himself to get into trouble to begin with?