I started playing UO when I was about 13. My parents paid the monthly fees for a few months, but I couldn't convince them to keep paying for it. ("Do your homework, Sean!") Frustrated, my friends and I found the free, independent, emulator-run servers and started playing on them. Finding the nuances between the free servers (called "shards"), we settled on one in particular and stayed there for the next few years.
I got bored with playing pretty quickly -- I was never really good at games that required skill or huge amounts of time investment. I started talking with the shard staff, gained their trust, and "graduated" into being a moderator for the server when I was 14 by doing some minor policing (resolving disputes, busting people leveling up with macro programs like UOAssist, etc). I could warp anywhere and got to choose special colours of clothing and had this awesome counselor robe. Looking back, according to [1], I best fit a "socialiser" player.
Then I got bored of simply moderating. There were a backlog of feature requests to the lone shard developer, who was busy trying to, y'know, have a family and work his day job. At the behest of the Italian couple who ran the server, I began developing for the shard when I was 15 (having not programmed before). I did lots of simple things like create weapons and funny items with special abilities [2], my favourite being a bag of skipping stones. Then I started doing more complicated things like recreate Triple Triad from FFVIII [3], and useful things like remake the guild system.
These UO people were my friends, we hung out on IRC every day, and even did a couple of international Secret Santa gift exchanges.
I got bored again around 16 or 17 and forgot about UO, but it had a lasting impact on me:
At 18, I began studying computer science. I realized that the code I wrote to sort a list back in the UO days (not knowing any better) was actually the standard sorting algorithm, bubblesort.
(actually relevant to the link -->) At 19, I started to learn about economics and related it back in a very real way to my days playing UO. I was tempted to go back and try to design a method of distributing wealth that reduced inflation.
And by 20 I realized I was a very good programmer. (I'm now 26.) I still attribute UO at the Italians I've never met in person for that.
[1] http://www.mud.co.uk/richard/hcds.htm
[2] http://neverlands-library.com/index.php?id=4lvl - can't believe I found this / someone documented this