I started having mood issues when I was 15 I believe, possibly before, and developed and ED as an unhealthy coping mechanism. If I could go back, I would make sure I had a therapist who could help me (mine kind of just didn't know what to do with me I guess, and I didn't seem that bad as she was just like "guess we can see how you do without therapy" even though we made no real progress) and I would possibly see a psychiatrist and try meds. Even if I had had a bad reaction to an antidepressant back then, at least I might be on the right path to a diagnosis.
I tried to manage on my own and did decently to deal with my moods for 16 more years until it got really bad. I just kind of wish I had realized and received that help as a teenager/in my 20s as I think it would have made a difference in my life. I have had many great things in my life, and am not complaining, but I don't really feel there is a "before" time for me that I would want to return to, except maybe when I was a kid.