Definitely by 14 with significant anxiety and perhaps some depression. It was bad enough that I finally told my mom (after a period of secrecy) and she took me to the doctor. He prescribed Buspar (buspirone). At 15, it worsened more, then turned mixed. The school advised my parents to take me to a child therapist after a rather traumatic event at school. By 16, it was a purer form of mania. I decided to "find God" then soon after joined the Roman Catholic church. It all eased when I was 17, and I showed only occasional milder symptoms until short periods at 20 and then 24. ["Milder" may seem significant to some. It's all relative.]

It's hard to say if I had any bipolar symptoms before 14. I certainly had periods where I exhibited some bipolar symptoms, but not severe ones. I remember feeling elated and grandiose as a little kid. I was called gabby by many people and scolded for talking over them. I had temper issues sometimes and brief "down and anxious" periods. I was definitely distractable a lot and the "star of the show". Was that normal in childhood or part of my young personality? It's hard to say. I guess it becomes a disorder when it becomes dysfunctional enough to really affect one's life in a noticeable way.