General questions:
1. Sophomore year of high school.
2. At the same time. The only reason I started getting into music seriously was because I decided I wanted to be a composer and I immediately started making haphazard attempts at it when I started learning theory.
3. I write the basic stuff down by hand, and then fine tune it all once I've got a Finale document going.
Composers who are self-taught (i.e. you had no private or group lessons for specifically composition, aside from a composition based assignment in a music class or ensemble):
1. I'd say it's circumstance. The fact is, I've never had a composition class. I learned a lot of what I know while in high school with the help of the band director. I wasn't in band, but he taught me music theory from the ground up since I knew hardly anything to start with. As far as college, I've been in and out of school and have yet to take an actual composition class.
2. The hardest part of having to learn everything myself has just been that I have hardly anyone to talk to about what I'm doing or how to get better. This site has been a great source of input and I do know a few people who I can talk to about music face to face, but not having many musical friends anymore really makes it harder.
3. I wouldn't know how to answer this question accurately, but I like to think it's ultimately led to me being able to make my own decision about what kind of music I'll write and how I'll write it.
4. It sounds both fun and intimidating to have a real teacher. It could be rigorous and help a composer grow faster, but I also worry that I'd get one of those teachers who doesn't want you composing music that isn't fit for the new age of atonality and experimental stuff. I'm sure there are lots of nice encouraging teachers out there too but it's hard for me to dismiss the more negative possibilities.
5. I'd guess it would depend on when I had taken them. If I'd started taking them earlier on and had them consistently then I guess it would be a tradeoff. Faster growth in exchange for a different course of development. I wasn't really the kind of person who could make strong individualistic choices easily. If someone were there influencing me I'd just end up being a pastiche of them. Recently I've made choices about who I want to be as a composer going forward and they've been in large part based on the fact that I don't have a teacher or training. I'm just going about cherrypicking elements of music as I come across them in life without having any one consistent source of inspiration or learning. It's been slower, harder, and more haphazard but I think it was for the better. Of course now that I've reached this point where I'm getting better at carving out my own character, I think having a teacher would be less likely to leave too heavy an impression on me and I'd welcome some learning (though the 'circumstances' haven't changed so that's unlikely to happen yet) and professional help.
I could be wrong about any or all of this. I don't know, I'm just saying what seems apparent to me.