I believe Giacomo 1 hit the nail on the head. With aging boomers getting back into biking, I've noticed a marked increase in the number of riders on the trails I frequent. Although some of the increase in accidents may be coming from seniors pushing the limits, there are other factors that come into play.
For the last ten years, I have been riding with a group of seniors who are non-competitive. We ride mostly on fairly flat trails but also do hilly routes and on occasion mountain bike trails. Most of the riders are now in their 70s, 80s and even two in their 90s. I've witnessed numerous accidents, at least 2-3 per year, and have some observations to share.
Some guys are just accident prone whereas others have never fallen. Those that tend to fall tend to have slower reflexes or exercise poor judgment. They can't think quickly enough to avoid potholes or negotiate ruts. A common problem for those who ride clipped in is difficulty in disengaging when they come to a stop. I've witnessed two broken hips this way.
As they have aged, some of the guys become unstable due to balance problems. This has resulted in several of them giving up on cycling altogether. One of them switched to a trike but then subsequently suffered a stroke. He still rides solo but can't keep up with the group.
As our biological clocks keep ticking, there are inevitabilities that need to be faced. The older riders who have never fallen appear to be more in control and seem to know their limits. Some of those who are more accident prone are just clueless. You have to give them an E for effort, but in situations where riders are bunched up, they can be a menace. I suspect that the highly competitive seniors will retain better reflexes and exercise better judgment longer than most, but at a certain point age happens.
One of our 90+ riders is still remarkably strong. He did a lot of touring in his younger days and I believe that is a factor in his biking longevity. I've seen him go down a number of times but he just gets up and climbs back on the bike with minimal fanfare. He's a joy to ride with.