I had a great childhood. Amazing parents. A good upper middle class life. Good friends. No real trauma to speak of at all.
By 12, if I'd shared with a doctor what was going on in my head they'd have slapped me with the depression label. I wasn't healthy. There was no event or trauma. This shit is genetic- I can tell which relatives on my Mom's side have the demon riding on their shoulders. They, like me, were born with it. I'm sure they can see it on me too.
Trauma can indeed trigger it, but there are a lot of other ways. Don't presume that just because someone suffers from depression that they have been traumatized, nor that someone who hasn't been traumatized can't suffer from depression.