Biggest effect is that your standards get higher. As a kid learning a game just meant learning to play it at a kids low skill level. Kids till spend years to get good. But as an adult who has played a lot of games you have gotten pretty good at many of them, then when you play a new game getting to that level again will take a lot of effort and time.

For example, you see the world records in factorio aren't held by kids. Kids has better reflexes which is crucial for some games, but their ability to learn isn't much higher than a young adults.

While possible, I am not so sure of that. I am in my early/mid 20s (so hopefully no major cognitive decline!) but I still find things taking longer than expected. Part of it may be the work I’m doing today is tougher in some ways. I also have adhd though so that may also affect this.