Age: This month's quiz is timely with many gulls in the northern hemisphere now beginning to replace flight feathers. Undergoing a complete molt, this individual has renewed most of its primaries which have an adult-like pattern to them. Overall the upperwing and tail are far from "definitive adult" and so it is perfectly fine to assume the newer flight feathers that have come in are 3rd generation. Thus, this is the 3rd plumage cycle and the complete molt here is the 3rd prebasic molt. The old retained primary (p10) is 2nd generation.
Identification: The brownish cast to the upperparts impart a darker feel than is typical of this species. Focusing on the new primaries, it is apparent that we're looking at a paler gray species. The bill is on the heavy side, but proportionately sized with the head. The eye appears dark, but zooming in shows much speckling and paling -- this helps rule out California Gull. The wide and complete tailband, and bill pattern, are wrong for a Ring-billed Gull, among several other features. The only reasonable identification for this pink-legged, pale, large white-headed gull, is Herring Gull.