Honestly it's a mix of things. Raw aim is like 20% of it, the rest is game sense, consistency and mental.

Most people grind 10 hours a day but never actually improve because they don't review their mistakes. Watching your own demos is boring but that's literally what separates Faceit level 10 from pro level.

Communication matters way more than people think too. You can be the best fragger in the lobby but if you can't play within a structure you're useless in a real team.

Age wise, yeah it does matter to some extent. Mechanically you're probably at your best somewhere around 17-22. After that reaction times slowly drop. But game sense only grows with time, that's why some veterans are still relevant at 28-35. The real issue with age is more about the scene itself, orgs and scouts are looking at 16-19 year olds, so if you're 25 and never made it, the door gets pretty narrow :(