http://www.mbr-pwrc.usgs.gov/id/framlst/i0540id.html

Young Ring-billed Gulls are a mottled brown, much different in
colouring from the adults. With each successive moult, or shedding
of old feathers, they lose more of the brown and develop more of
the white, grey, and black patterning. They attain full adult
plumage when they are three years old.