Not that I'm questioning anyone's aging of the 3rd-winter
Glaucous, but in my experience it seems most 4th-winter large
gulls still retain minor signs of immaturity such as those described,
so sometimes I've found it hard to be sure on the age of older
sub-adults. Since large 4-year gulls don't have a full molt in their
hatch year, 4th-winter birds have only undergone 3 full molts,
and I think most are still distinguishable from adults until at least
their 4th full molt (4th-summer going into 5th-winter).

Just something I've found interesting that isn't mentioned in most
field guides.

Cheers,

Phil