I bought my first O scale train as an adult when I was 25. Now that I’m 10 years older, I can afford to buy higher quality locomotives and now that I own a home, can actually build a "permanent” layout.
I prefer O scale because it’s easier to handle (compared to smaller scales), seems to have more details (or maybe I can’t see details on smaller scales), and it was the scale of my first set that was around the tree.