Plumages and ages

Juvenile
See here.

Post-juvenile
See here.

Young
See here.

Immature
See here.

1st-autumn
A bird it its first autumn, 2-5 months old. It refers to its age and not its plumage.

1st-winter
An age category usually referring to the plumage following that of a juvenile bird, usually gained through a partial or complete (in some species) moult in late summer/autumn of the 1st calendar year and is worn until the next spring of the 2nd calendar year.

1st-summer
An age category usually referring to the plumage worn at the approximate age of one year and attained in late winter/spring through partial or complete moult from 1st winter plumage; worn into the next moult usually in late summer/autumn of the same year, replaced with the 2nd winter plumage.

2nd-winter
A complete or partial moult in late summer of the 2nd calendar year and worn until the next moult, it is followed by the 1st-summer plumage