Age: The above bird looks typical for an adult, large white-headed gull, while the bottom gull appears to be a typical 1st cycle.
Identification: Beginning with the adult, the slaty gray upperparts point to a black-backed. The bulbous banana-yellow bill and clean white head have the stamp of a Western Gull (although Yellow-footed can't be safely ruled out without leg color). The 1st cycle doesn't appear to be the same species based on the thinner and smaller bill, as well as narrower wings. The lack of an inner primary window, with double-brown "secondary" bars, and sharply demarcated black tip to the bill, give us every reason to believe this is a California Gull. The medium gray post-juvenile scapulars that have come in also support California Gull.
June's quiz birds are indeed an adult Western and 1st cycle California Gull.
Santa Cruz County, California. January.