Being in your 30s and 40s isn’t old as you probably picture. Now, if you have a family and a mortgage you’ll have more responsibilities. And your body might not be what it once was. But mentally, I don’t feel different. I completely forget I’m not the same age as people in their 20s when I interact with them.
To answer your question, I suggest sorting the things that you only can do when you have fewer responsibilities (traveling for a long stretch, moving to a new city, quitting your job, working out and exercising). Do those things now, and remember you can do a lot in the decades to come, too (e.g., I didn’t start going to concerts until I was 40)
(Also, I sincerely apologize to everyone in their 20s who is working their ass off just to get by!)