I wouldnt say theres a definite answer there.. just general guidelines, some beardies will develop quicker than others.
I would say at between 1-2 months you cant really call them hatchlings anymore so they are juvi's.
at 6-10 months old OR Once they hit around 14 inches they must be considered sub adult.
At 20 inches long, or a year and a half i think urv gota say young adult.
and after they have bred once or twice, hit 2 foot OR hit 2 years old i reckon they must be adult.
Thats just me having a quick speculation really though to be honest.