So now you shoot people in the back? :p
It's all about memorizing maps, player patterns and behavior. I'd say 80% of players do the same things during the same senarios. Once you memorize these things, no reflex will be able to compete.
Having said that, at 43, you should still be able to compete reflex wise if you want it bad enough. This isn't a real sport. The body breaks down before the mind and video games only use the mind.
You sure about that? I can cycle 130km in 5 hours, yet holding a thought in my head with my kids around is impossible! Only having time to game when I really should be sleeping doesn't help ofcourse. I've nodded off during online races before. (Actually I've also nodded off while cycling ending up in the ditch)
Anyway when I played HL en UT it was about the same things. Know the layout, know where the power ups are and when they respawn, know where players like to hide etc. Maybe my reflexes can still compete when well rested, yet I don't have the patience anymore to learn mp maps inside out and play it every night to get perks and what not. I guess the appeal of PUBG is, only 1 map to learn, no rpg stuff (afaik), level playing field, and plenty of luck to keep it interesting.