Age: Profile and stature suggest a large-white headed gull. An open wing would be helpful for such a plumage aspect, but the overall muted and plain pattern to the tertials and upperwing coverts, as well as the amount of adult-like gray on the back, suggest a 2nd cycle type gull. The primaries, particularly p7-p8, are gently rounded at the tips, which should help move us away from 1st cycle. The bill pattern, with yellow on the nail, is also more expected on an individual beyond its first cycle.
Identification: Beginning with the primaries, we note a dark brown coloration -- not black -- with prominent pale edging, especially on p5-p6 (closest to the tertials). The tertials are a couple of shades paler, encircled with a pale outline. The coverts become a shade paler than the tertials and have a noticeable frosty look. And so from the wingtip to the lesser coverts, the bird gradually goes from dark to pale in an even manner. Structurally, the bill is relatively thin with no noticeable buldge at the gonys. The head is nicely rounded, and the head and breast markings are more smudged than streaky. Also helpful here as a supporting field mark is the vividly pink legs. All of this points toward a white-winged gull -- too lanky and small-billed to be Glaucous-winged, which leaves us with Iceland Gull.
Additional Comments: Note that Glaucous-winged x Herring Gulls can approach this plumage aspect. Those hybrids tend to have a heavy-chested appearance, as well as more body behind the legs and slightly shorter-looking wings than this. But surely there are instance when these two -- Thayer's and Glaucous-winged x Herring hybrids -- can be difficult to separate.