These are general rules, not hard and fast in my experience ...
Chick - nestling - hatch to fledge age.
Eyas - same as a chick, not used so much by biologists in my experience and not a term I use. Sort of like tiercel and falcon for male and female, terms that require explanation when they are used.
Fledgling - when chick takes first flight (fledge) so they are now fledglings; remain fledglings until they have the whole flying thing undercontrol, then usually shift into being juveniles.
Juvenile - usually used when birds are more independent - hunting, roosting, moving further afield.
Young - also a term for the offspring of a year, can be of any age - chick, fledgling, juvenile.
Immature - term that gets used for birds that are after hatch year (January onward) in age but who are still behaving like juveniles i.e., begging for food from adults
Sub-adult - young non-breeding peregrine, may show some juvenile plumage or may not, behaviour is adult, but not breeding due to age. Some just call them non-breeding adults.
Adult - old enough to breed, may or may not be breeding or have bred.
Or at least this is how we use them ...
