The death of my mother when I was 11 is probably had the biggest effect, not just in terms of the emotional trauma that one would expect but also I think to some extent it led to me developing differently as a teen that I would otherwise. I reckon I would probably have been more socially active as an adolescent had she still been around, but of course I'll never know.
Another very significant moment, which probably sounds very trivial to most people, would be me taking up an online computer game called Quakeworld during my first year at uni. I went from being a straight A student at school to a lazy bum at uni who spent countless hours each week playing Quake and ended up with a rubbish degree (Sujoy Roy, anyone?). The funny thing is I still don't regret it per se, since it was immense fun and I made some good friends along the way. At one stage I was probably one of the top 3 teamplayers in the UK and I even travelled to a LAN event in Sweden. It definitely changed my life and was the driving force behind my continued interest in computers and the internet. It has also brought me into regular contact with fellow gamers from at least 20 different nations and even taught me quite a few foreign words and phrases in Swedish, German, Dutch etc. Culturally I am far more aware than I used to be (aside from a school trip, I didn't venture overseas until I was 23).
More recently getting together with my gf just over 2 years ago has changed my life yet again. It's given me the motivation to get a better job, buy a house and ease off on the gaming a bit.