A fledgling is a baby budgie just out of the nest and its first couple of weeks out and about.

At its first moult it is a juvenile.

A budgie is deemed an adult past a year of age.

Show budgies are deemed adults by the legrings if they wear the year before's ring they are then an adult and younger they are classed as "ÿoung".

If in reference to breeding you are looking for the definition of an adult for breeding, its preferable that they are a year or older.