system
A chat on 4chan
human
Probably about 2001 or possibly 2014, oddly. 

In 2001 I started middle school and moved farther away from the Christianity I grew up in. I ended up having a lot of trouble in my teen years becoming an extremely edgy teenage anarchist. I went 0-100 from being just above a Mennonite to listening to Eminem in a year in 2001. So if I could have kept my eyes on God and stayed the gentle and kind person I was, I would have been better off. I think all I needed then was trying a different church. My parents also divorced then. I definitely wouldn't have been able to stop all the events that happened around me due to my parents and church collapsing, but having my current perspective as a kid would have helped me out a lot, including how to handle human relationships and win friends and influence people rather than being edgy and a dick for no reason all the time in an attempt to be 'cool' and make lulz. Missed many girls from no confidence, even when they initiated. Also should have kept studying Japanese. 

2014 I'd pick as I was very close to being a high level athlete and really accomplishing a lot of my dreams like going abroad to Japan/Asia longterm and I threw it away a year later in a long distance relationship with a girl who abused me and treated me like dirt. I accepted as it was my first ever relationship and I was a KHV prior. After getting involved with her I abandoned a lot of my dreams, especially my main one of being an athlete, got fat, drank way more, worked way more hours, and abused my body with too much caffeine and possibly ruined some internal organs due to it and shortened my lifespan. Also she is not a believer, either. So turning back time to there would really instantly improve my life. This is not just with my one sport dream, but college, investing, etc, are all things I'm starting now at 30 when I could have had a PhD and a reasonable amount of money from literally investing in anything at all if I just did what I'm doing now then.
gpt
It's not too late Anon, go get the life you want and deserve!