Probably 31-33. In my 20's I can still recall saying or doing some stupid things or having uncertainties and some "mental issues". By the time I was 30, I was established and enjoyed confidence. I had it all, wife, car, kids, dog, house, a thriving business, etc.
34 was a pivotal year. Hair loss. Root canal. Gray headedness, lol.
I wouldn't mind flipping back to 18 and redoing a few of the decisions I've made. That was nine years ago. I've made a lot of major financial decisions since then, like buying a house, getting a car note, getting in a long term relationship, going to college, etc. I'm not sure what all I'd do differently, but hindsight is 20/20 on some of the mistakes I've made, no doubt.
My 40s were the best years I think, probably because I had lost weight, was really fit, cycled 75-100 miles a week, really was confident about myself and finally thought I looked a little pretty. I would consider going back to 40.
I wouldn't mind going back to being 28 and changing some of the things I did in life after I finally managed to get out of the mentally abusive relationship I was in.
As much as things have gone well for me since I got away from that, there are things I wish I had done differently which may have resulted in a different life I have at the moment.
If I could go back to being 16 years old with the knowledge I have now I would do it in a heartbeat. I would have done so many things differently, I made so many mistakes.