Somebody correct me if I'm wrong, but I believe they're considered hatchlings/juveniles until about a year old. After that you'd refer to them as a yearling or sub-adult, and they are usually full-grown adults by age three. I'm not sure about the cut-off between hatchling and juvenile, but I would only refer to mine as hatchlings for about 3 months. Hope this helps!

P.S. Full maturity generally comes earlier for male BPs, but you probably still wouldn't call them an adult for 2-3 years.