1. Playing fewer video games in university, and focusing more on studying so that I didn't fail my final year. I started university three years early, which has opened doors, but having a degree would have opened more. Also, paying more attention to electromagnetics and thermodynamics.

2. Continuing to train for Tennis. You really can get too old to do this at an elite level. I love training, and I love being in the zone: perceiving / acting / crushing the opposition. Sport is a natural fit for my attention, physicality, and aggression, whereas long-term projects are more of an uphill battle.

3. More travel, although I'm not really that interested in individual people, so perhaps it would have been wasted. To the extent that I'm interested in other people now, it's to figure out what they know that I don't know and how they acquired that knowledge or experience.

Don't get me wrong. My life is pretty sweet, but I do have regrets. It's more that I'm realizing the difference between turning 30 and turning 40. At 30 you can pretty much do everything you could do in your twenties. At 40, doors are closing, esp. regarding physical injuries and recuperation.

I just kind of always assumed that I'd have a PhD by 25, be a concert pianist, a professional tennis player, and a software millionaire. I've had brushes with each, but so far no completion. Boo fucking hoo, right? Unrealistic expectations.

Thanks so much for sharing. At 25, I've been feeling a teeny tiny version of that (I had a lot of free time in my teenage years that I frittered away, and wish I had spent it developing skills). But I realize that 10, 20 years from now I'm going to look at today the same way, too.