It's worth looking up reported happiness vs age. Happiness peaks in your 20s and then steadily decreases until you are in your late 40s. Then it rebounds and keeps increasing until end of life. So that's one data point.

What I learned is this: the number of responsibilities continues to mount. The difficulties of life become more and more real. You experience your own work stress, stress over kids, marital stress, health concerns, family deaths, financial stress ... And it all weighs on you. When I was an undergrad I was busy studying. I had things to do but I had NO idea that it was also as much discretionary time as I was ever going to have.