I rank them as pippling, hatchling, yearling, subadult, and adult.
Pippling - pipped the egg, but not quite hatched yet
Hatchling - from hatch until the next breeding season begins
Yearlings - year old (give or take) until the following breeding season
Sub-adult - older than a yearling, but not breeding size (some corns never get this title)
Adult/Breeding Adult - has reached breeding size
I'll use Juvenile for around the older hatchling stage through small sub-adult stage, but it's less specific in my vocabulary and kind of a catch-all for "not fresh hatched, not adult".
There aren't any set in stone definitions, but most people use some similar system.