I started stuttering around first grade. It started very gradually and developed into full blown blocks, repetitions, etc by the time I hit age 10 or 11, at which point I started going to speech therapy. My personal theory is that whatever caused it to start happened, but then was exacerbated and intensified by my very type A family. When I would have trouble speaking (up until it became enough of a problem to go to therapy, and even then), my brother in particular would hurry me along, finish my words, and just get generally shitty with me about it. As far as triggers go, mine are completely random. Sometimes I'm perfectly fluent when angry, sometimes its the exact opposite. Sometimes there are particular sounds that I just cannot say for the life of me, sometimes I can say those sounds with no problem. When I went through puberty it was a tiny bit more predictable, but now that I'm an adult it just happens when it happens.