I don't know how close to the norm my definitions may be, but here goes:
Neonate: up until it about doubles its hatch weight (usually about 3-4 months).
Juvenile: between neonate and sub-adult (up to abt 18 mths).
Sub-adult: the year before it will achieve full breeding weight (abt age 18-30 months).
Adult: the year it first breeds (usually over 30 months).
The above would generally apply to North American colubrids (Elaphe guttata and Elaphe obsoleta species, and probably also to Lampropeltis). It may be shorter for smaller animals, such as garter-snakes, and may be longer for boids. Since I'm unfamiliar with anything outside the Elaphe family, you may want to ask someone who's breeding the same species that you do.
"Nothing is so firmly believed as that which we do not know."
<i>H. L. Mencken</i>