There's plenty of impact when you hit the road.
A guy who has raced many years will not crash as much as someone with less experience, but the body definitely doesn't rebound as quickly from a crash as it ages (ask me how I know).
I just came onto this thread and don't have time to read the whole thing, so someone may have mentioned this already, but one thing to consider is eyesight and reflexes, which do decline precipitously after approx. age of 45. This is probably not as much a factor in road races as in crits, but still something to consider in a dense pack.
Cardiovascular is the main problem at this age. No matter how healthy you live, the arteries get increasingly narrower as you age. I suspect that the geezers who are doing well in races put a lot more energy/time into training to compensate for this, which doesn't leave much time to pursue much else.
On the positive side, I've found is that the legs remain remarkably strong, even at my present age of 50. They seem to remember the "old days." I don't know if I could hang and bang with the field today, but not too many riders pass me in Central Park, and when they do I can usually catch them. So with good technique, good tactics, attention to large-muscle mechanics, hours and hours of training, an older guy can probably remain competitive, even in the A races.
The question for me though is why would he want to put himself through this kind of misery at his age. . . getting up at 4:30 a.m. (in NYC at least) shooting his wad, and then being stiff and sore for the rest of the weekend.
Personally, I'm happy just to loop the park every other day or so and "surprise" the younger guys for a lap or two.
My two cents. . . I guess this was long-winded.