I don't understand when people stop doing things they enjoy because they feel like they're "too old" to be doing them anymore. If it's for a health reason that you have to give it up, that's unfortunate but you taking care of you is more important than any fun thing you do.
If you're just quitting something you still like doing because you feel like you shouldn't be doing it anymore because you're "too old" to be doing something immature or silly, that's nonsense. You need things that bring you joy, even if it's just thirty minutes with a video game or a spirited game of dodge ball.
"Being an adult" and "growing up" shouldn't mean you also have to give up things that bring you happiness. I know several people that think I'm a rather immature 30 year old and honestly, I couldn't care less. If it makes me "less of an adult" to anyone because I still color in coloring books (even before my therapist had me pick up art therapy specific coloring books) or thinks I'm "immature" because I've organized a water balloon/water gun fight with other "immature adults" then cool, you're certainly entitled to your opinion. I however, don't currently and likely won't ever agree with that sentiment.
I don't know where people think getting older and becoming an adult means you have to just stop having fun. Being an adult is stressful. Working so you can pay bills and make a living is stressful. Being an adult in itself isn't that fun. It's full of responsibilities and things that are inherently not fun. That's not a bad thing, that's just the course of life. I'm glad I'm not a child anymore. I love having my own place, cooking my own meals and creating a life for myself and my partner. As stressful as adulthood can be, I'm fine where I'm at. I also just refuse to believe that I can't also still have fun and fun as an adult doesn't have to be dinner parties and happy hours and trips to the museum. I do love all of those things as well but there's also nothing wrong with having a water balloon fight or planning a trip to a paintball course to climb obstacles and fire off speedy little balls of paint at a high rate of speed in hopes they'll explode in bright colors once it hits it's target (AKA your friend.)
If you don't enjoy things, it's cool to let them go and not do them anymore. There are some things I used to enjoy that if you presented me to them now, I'd decline because they don't hold any appeal for me anymore. Some things you do grow out of or simply lose interest in, that's also just how life goes. We're growing and evolving and that includes our interests and hobbies, too. Just don't deprive yourself of things you still enjoy doing because you feel like you're "too old" to enjoy them anymore.