Here's a hot take: you are in charge of your life. (Because this meaning of "life" is a social construct.) It isn't "life" that changed, it is you who became boring and dull. I guess all work and no play makes one a dull boy.
Watch a Disney movie from the times they still were good:
I agree, there is absolutely nothing to do after 31. You already experienced everything you can, your life turns into the wageslavery and sitting near PC. Also there is that middle ages crisis when you think that you made a lot of mistakes in your life and you failed at everything.