Welp... not a lot.

The reason being that by 30 I was already married and in my first owned home (with mortgage, but still). The plan was kids, slightly bigger house in a slightly nicer neighborhood, and otherwise wing it.

There's nothing I could change substantially that wouldn't involve sacrificing the things I wanted and planned for or sacrificing principles which I stood up for once or twice during my 30s.

I did have a couple of minor medical conditions that could have been resolved by a trip to the doctor rather than taking over a decade to realize I'd misdiagnosed myself. I suppose I'd take care of them earlier. Maybe I'd have gone a bit wilder with the missus before we were tied down by children.