International B and Grand Prix events - 15
International Championships (Euros, 4CC, Worlds, Olys) - 16
Actually, for International B and Grand Prix events, the rule is 14 by the previous July 1. Skaters in this cohort with mid-summer through late-fall birthdays will be 15 during the fall competitions; skaters with winter or spring birthdays could compete at these events when they're 14.
For the senior championships, it's 15 by the previous July 1. So by the time these events roll around, later in the season, especially Worlds, most of the affected skaters will have turned 16 already, but a skater with a June birthday could be only 15 and 8 months at the Olympics in February.