Alright, I'll give two, because I believe both are equally important. One is music related, again, while the other not so much.
1.
Picking up a guitar and learning to play it.This one may come across as weird for a pick of a 'defining moment', but it really has been. Before I played the guitar I was a member of a musical society that trained children's bands. I was in a band there from 9-10 years old onwards playing the saxophone in a band with other people my age and we were coached by elders, like many other bands consisting of children. I was never too serious with music and liked going there, but basically I never really practiced (to be fair, I could play the saxophone quite well, at least well enough to improvise random stuff in all of our songs and I was musically 'talented', however much I hate to use that term), but I enjoyed it nonetheless. When I was 15 years old I was still in a band there, but I found a guitar in our attic. I went to a shop, bought new strings, a guitar pick and found a beginner's book and starting teaching myself.
My interest in music went through the roof. I discovered music that I liked and I played for 5-6 hours a day. From 'not really that interested in music' I became an addict, searching new music, developing my own taste and learning the guitar, music theory, rhythm and all that. Before I started playing the guitar, I thought I wanted to study Biology when I finished high school, but I was determined to go to a conservatory with my guitar, all to my dad's enormous disagreement. At my music society I became a band coach as well when I was 17, teaching other children to play in a band, as I was taught there. In the end I didn't end up at a conservatory, I'm a musicology student now, but I still play the guitar a lot and I play in different bands as well. I'm not a member of the society I grew up with, because I moved to attend my university, but I owe the people there, but mostly my picking up the guitar for my music taste, my musical abilities and most of my adolescence so far.
2.
Moving to another city to attend universityAh, yes. The classic. Child becomes a grown-up. Well, indeed. Moving out of home was probably one of the best choices I've ever made. I learned to take care and look after myself, to live with other students in the same house, I got a goal in life (study) and I've made lots of good friends. I'm currently at the start of another landmark, as I will become the next treasurer of my study association and I'm a pretty good student so far, doing well both in and out of university. I'm very happy at this point in my life and as far as I can see, things will only get better.
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That was longer than I thought it would be, I hope it wasn't a boring read