I was diagnosed with MDD at 14. The only trigger I know is that I had switched from a small school (about 19 people) with people I had known since first grade to a performing arts high school that had about 350 people. I was completely culture shocked.

That said, the episode was explosive. I started having to leave school fairly often because of my delusions and fears (ie: that my geometry teacher was an alien). I was a complete mess and the antidepressant I was given caused me to become manic. And that mania, when it showed up, was completely bizarre. I was outside at 11 PM doing cartwheels and I'm fairly certain my mom was afraid I would attempt to fly. I tried other antidepressants and it wasn't until I found a new psychiatrist (at 15) who accurately diagnosed me that they knew what to look for. I acted out frequently. I would regularly leave my geometry class and hide in the bathroom.

I also was leaving school, or melting down, or behaving irrationally so often that my parents pulled me out my sophomore year and I did a "home hospital" type of home schooling. I spent the rest of my high school career at a school that incorporated both "regular" students and those with disabilities. There, I could interrupt class by singing I Got Friends in Low Places and have it received with uproarious laughter and singing along instead of being disciplined.

I feel extremely blessed that they found it when I was a teen despite having such disbelief over several years.