Age: All 3 birds are in high molt, with a mixture of old and new upperparts. The visible primaries on the rightmost and leftmost birds appear to be 1st generation (note the primaries on the center bird aren't visible). Thus, it's safe to say these large white-headed gulls are roughly 1 year of age.
Identification: Both individuals with bi-colored bills look like typical 1st summer Herrings - size and structure rule out smaller species such as Ring-billed, and the thick bills and heavy chests seem to rule out California Gull. The left bird - with a mostly black bill and attenuated look to the rear - is different.
Here's a slightly underexposed image of this individual:
The upperparts are genuinely dark with dark centers. Note the bill is relatively long, but straight, ending with a slight droop. The striated neck markings, and dark stippled tips to the new wing coverts, recall Lesser Black-backed Gull, and that's what this month's gull was identified as.
This first summer Lesser was photographed in Sheboygan, Wisconsin. June.