The cause of total fatal accidents in genaral aviation are caused by two primary factors involved, the pilot and the machine. 5% can be traced to the powerplant, airframe, equipment accessories or to the inadequate maintenance. The second primary factor is the pilot, he is physically fit by law, he is mentally capable, he is responsible for 85% of the total fatal airplane accidents. The remaining 10% are undetermined an miscelleanious. The most dangerous characteristics an airman can have is false pride, he is afraid other people will recognize his limitations. Exhibitionism is a dangerous personel characterism when it affecting the decisions of a pilot. There are among us those who rebel against the rules refusing to recognize they are a tool of common sense, such a pilot is a VFR pilot who flies above the cloud deck without ground references trusting blindly in luck. Our personel characteristic affect so often our decisions regardless the age.
‘Old’ these days seems to start at 80 on average, and it seems that a person needs to be unfortunate or really abuse their bodies to be unwell before 60ish. I do meet a number of very sharp 90-somethings, though by this age there’s usually something important failing.
Just got checked out on Zlin Z43 by a 78-year-old instructor. The guy is sharp as a tack. A few years ago he stopped renewing his test pilot and instrument ratings because he wasn’t using them, and remained “only” an instructor and a chief pilot of a flying club.
…and this pilot gives a Churchillian two finger salute to hanging up the headset 
I’ve had one experience with an old and no longer competent pilot. Otherwise, the guy I continue to learn the most from is 74, has flown more planes than anybody I know, and is now flying his RV probably 125 hrs a year. Another guy who I think is exceptional is 85 and as active as ever. Those two flew every day for years when they were younger and I’m certain it’s true that the accumulated experience is important in compensating for whatever happens with age.
The one example was either amusing or a little scary, depending on your POV. A friend’s father took off, got lost in the local area, squawking at 7000 ft over the airport without knowing his location, and displayed all of that in communication with the tower on his way back. In the meantime, he’d busted Class B airspace by 2000 ft vertically. In the end he came back and made a perfect landing in his slightly hot tail wheel homebuilt. His son took the keys, and called FAA to discuss. The conversation was very short, and along the lines of “I’m his son. Sorry, he’s 85 years old, flying since age 16 but he’s done, I’ve taken his keys”. The response was “OK, thanks for calling, please handle it” and that was it. He died at 90 and hand propped my plane until almost the end. His plane flies on decades after he built it, now flown by his son.
In my observation, there is definitely an abundance of old and no longer competent pilots. Same problem in road traffic.
It’s not so difficult to extend a medical with declining mental fitness. They can concentrate and perform for the examination but not necessarily in a constant and prolonged fashion. Early dementia is not a constant condition, cognitive abilities alternate between normal and a danger to the public. Refusing to extend a medical is a severe measure, especially after having had the pilot as customer for 20+ years. That is not done lightly.
There is a 82 year old billionaire in my area who owns a large GA airport and flies his Falcon to commute to work. Alpha male par excellence. Apparently he thinks he’s pilot flying (LHS) but in reality it’s his employed pilots on the RHS who have to constantly save the aircraft and the souls on board. So the story goes…
A few years away, hopefully, but even with a clean bill of health I would plan to hang up my headset for P1 purposes on reaching 80.
There is a logical pitfall here: even if
accident reports are littered with age 70+ pilots,
I know someone who had a stroke in his 40s… you might be next, Achim. In life you never know what is just around the corner. Or get yourself a stent (even 1; with today’s dietary habits 3 seems to be the entry level) and your medicals suddenly go from €200 to €2000, for ever. Also IMHO your examples (obvious dementia) point to a very dodgy AME and the moment he stops doing medicals, 2 things will happen: the pilots will be in deep sh*it, and the just-retired AME will be in deep sh*it. But yes there are such AMEs around and I know of some. They can be useful 
but one is on borrowed time.
Anecdotally this proposition seems plausible but there is a massive amount of risk compensation going on as pilots get “old”. Also many pilots have “done it all” by then, done the “European touring stuff”, and now just want to go up locally.
I don’t think there is as much risk compensation with drivers because anybody who can get inside a car and put in the key can “drive”; in the old days people would be so p1ssed they would be carried to the car and somebody would start the engine Also you need a lot more IQ to pass the PPL exams.
As for proper studies which the OP asked for, I can’t think of a direct analysis by pilot age at accident time. It would also difficult to do “well” because of the pilot age distribution which basically starts to take shape around 50, so most pilots – who stayed in GA for a bit at least – will be quite a bit above that by the time they pack it in or have an accident. So most of the group will be “old” and then to get anything out of it you need to analyse it deeper.
I am 60 BTW, which I believe is the new 40
I’m in my fifties and wanted to write a question, but just forgot what it was…
