Take a look at the second picture here, for the kind of terrain she was training to run on:
I would tend to think that age affects reaction time significantly only after a person hits their 50s, maybe even early 60s depending on the type of sport. I mean people can still drive very well in their 50s and they also have considerably more experience, whereas a person in their 20s may take a stupid risk more easily. Professional athletes who are counted on to win games and bring in big money don't quit til their late 30s. JMO
ETA: Having said that, what does affect reaction times and attention is an empty stomach. I'm wondering if she'd had anything to eat before going on her morning run. Breakfast in their hotel was not served til 9.30, we know that from her bf who said he was waiting for her to return so they'd have breakfast. I know lots of people like to exercise first thing in the morning before they've had breakfast. I also know that whenever I've tried to do that, I felt semi-dizzy and got a headache for the rest of the day. Eight hours without glucose is too much, and then to go for a run still hungry on top of that? But there's no way to know what happened in her case.
If this were true, we'd see a lot more 30 and 40 year old Olympians. 40 year olds would as fast out of the starting blocks as the younger participants (they aren't, although certainly there are rare individuals still competing in their late 20's or possibly 30's). If we widen to include national levels of sporting events, it's clear that there are very few really speedy football players in their 40's. Or basketball players. Or figure skaters, for that matter.
Of course there are other variables (although if a person is routinely exercising before breakfast, that shouldn't have that much effect). Many people are athletes and do intermittent fasting, in fact, it's quite common.
Both of the Williams sisters have slowed done a little (and been bested by younger people).
The sport I follow most closely in this regard is climbing. I think Alex Honnold is taking fewer chances these days and shifting out of his free solo mode (for him, going super fast on a less challenging route than his free solo ventures is easier; it woudl be like Dr Christopher giving up mountain running to focus on speed in marathon running).
Here's a chapter on the topic:
Handbook of the Psychology of Aging
The authors claim that each decade makes a difference in athletic performance. My point is that a person attempting to beat their own personal best (set a decade earlier?) may put themselves at risk. While many people still run marathons or climb even in their 80's and 90's, their performance is not as good, and they need to account for that.
Making a mistake in grabbing a loose boulder can happen to many people, regardless of age; the most common feature of these types of accidents is being alone.
