Each keeper has his or her own ideas on this subject. I have hatchlings from hatch to a year, then yearlings until two years, then juveniles. Mine aren't adults until they are sexually mature at which time they are young adults. Once they have done the deed, they are adult.
Here is how I break it down:
Hatching to loss of egg tooth = Hatchling.
Loss of egg tooth to 12 months = Baby.
12 to 24 months = Yearling.
24 months to 14" = Juvenile.
14" to roughly 18" = Subadult.
18" plus = Adult.
This can get confusing when a fast grower reaches 14" at 18 months old… In those cases I just skip ahead. So I would call a 14" 18 month old a subadult, and skip the yearling and juvenile designations.