Age: This appears to be a large, four-year gull. Based on the rounded primary tips, muted pattern to the inner primaries and wing coverts, moderate gray on the back, and largely white tail coverts, we can safely assume this is a 2nd cycle. A 1st cycle holding on to all its flight feathers would likely show less gray on the back, and patterned uppertail coverts and greater coverts. A 3rd cycle would show adult-like primaries, or at the very least, adult-like inner primaries, which this individual does not.
Identification: Based on the darker gray scapulars that have come in, we can eliminate paler-backed species such as the Iceland Gulls, Herring Gull and even Glaucous-winged Gull. One may ask, how do we correctly interpret the gray on the back, and this could be done by judging the white on the uppertail coverts. The white on the uppertail is a true white -- not blown out and not covered in shadow. Thus, we can visually measure the contrast between the gray upperparts and the uppertail to get a sense of gray values.
This is a broad-winged bird with noticeable white tips across the secondaries. It looks too broad winged for Lesser Black-backed and the bill appears too heavy for that species too. Judging from the swollen bill appearance and smudged head and dusky neck markings, this bird best fits a 2nd cycle Western Gull, and is actually a fairly typical bird. A 2nd cycle Yellow-footed Gull at this time of year would show some indication of adult-like flight feathers and should perhaps average more gray on the upperwing coverts. The date and location also support Western Gull.