With Oldest, it was within the first few months. She was an extremely colicky baby, screamed every night. Later, she threw some prety incredible tantrums as a toddler, refused to go to bed at night, and got kicked out of day care at age 4. The first real "diagnosis" of "possible bipolar" came at age 11, after trying to jump out a window with "prednisone-induced psychosis" (she was taking it for her Crohn's at the time- she was diagnosed/hospitalized with Crohn's at age 9). Docs never did get her moods under control, she got only intermittent psychiatric help because they were too busy trying to keep her Crohn's at bay. She gave the nurses in the peds unit at our local hospital a run for their money; they sometimes had to call security on her when she'd go into rages in her hospital room. I spent a lot of time crying in the parents lounge down the hall as I listened to her screaming at the staff.
With Youngest, she started throwing massive, screaming, hitting rages at age 3, shortly after their dad and I split. She began wetting the bed on purpose (she'd call down to me, "mommmmy, I wet my bed!") , and it took hours to get her to sleep some nights. With the help of a good therapist things got better for awhile, until about age 9/10 when she began talking about hearing voices and going into rages again. I think it was no coincidence that that was when Oldest was at her sickest with the Crohn's; Youngest later said that she made the voices up. The rages were very real, though, and she was first hospitalized at age 11 after pulling a knife on me, and diagnosed bipolar as well.
Fun times.