Good question.
To me, a hatchling is just that, a new born leo, and before it's first shed.
A juvi would be when the leo starts to eat normal size food, and after the first shed. It stays a juvi until sexual maturity.
Not sure what folks mean by "sub adult" and adult. I dont make that distiction. To me, after they reach sexual maturity, they are adults, period.
Generally speaking, I consider a hatchling a gecko under 10g. Once they pass 10g til about 6 months (when all the dramatic color change is going on) I consider it a juvie. 6 months to a year is a sub-adult, and anything over a year is an adult.