For years people have been touting a line about losing react time when you hit 23-24 years old, no idea if it's apocryphal or not I haven't personally seen a study or whatever.
I lost interest in competitive / PvP games well before then, they could be right and I wouldn't know. It isn't a noticeable issue if you only play map pervert games.
I have 100% gotten worse at video games after 25. I don't think it's reflexes, I doubt my reflexes have changed enough to matter for the gaming I do. I know at least part of it is lifestyle. I can't put as many hours into gaming anymore, I'm out of school and tbh I'm a bit out of the habit of pushing myself mentally, which I think has made me a slower learner. Actually I'm glad I'm typing this out cause I hadn't thought about it this and I'm sure now that I'm aware of it I can turn the trend around.
Clicking mouse requires 82% more brain power than your average “musician.” Beethoven can write a symphony but he can’t hear footsteps coming up to knife him 🥱
I don't think reflexes decline, or speed. Fucking thirty year old Starcraft pros can and do remain top level despite their relative age, and we really don't see APM decline that much.
I think it's more that young players can bring a new way of viewing a game to the forefront that is more effective than the old way, and the true test of a pro is whether they can adapt to a changing meta or if they'll cling to the old ways until their entire playstyle is unviable at the top level.
This goes doubly true for games that are receiving regular patches, since the meta is shifting much more frequently. People who get good at the current meta will rise to the top, but if they're inflexible they'll be selected out once their playstyle is no longer top tier.
(as for non-pros, people over 25 probably are worse at playing games, but it's because they have more responsibilities that reduce their practice time.)