Im sure there have never been true hormonal tests to determine a realistic age...but I would assume that at about 1-1/2-2 years they are about as developed as they will be. Someone else can feel free to chime in.
They seem to reach sexual maturity much younger - around 9 months or so. But my males continue to grow in length and weight until they're about a year and a half. The really big ones sometimes top out at two years, though