Age
Both of our gulls appear to have mostly new upperparts, although notice some feathers have been dropped and others are actively growing (namely, lesser/greater coverts and lower tertials on the left bird). The tertial patterns appear to be 2nd generation on both individuals, but the outermost primaries that are visible have an old "brownish" aspect. These primaries are juvenile feathers (=1st basic). Therefore we can comfortably assume both birds are molting from 1st to 2nd cycle and are roughly a year old (2nd calendar year).Identification
The disparity in size among these two individuals will most definitely be a deciding factor in assigning labels. The gull on the left has a genuinely hefty bill and legs with remarkable girth. The head is proportionally large and the body is reminiscent of a duck. All of these structural features, along with the ghost-white head and solid dark upperparts point directly to Great Black-backed Gull. The bird on the right looks like a mini version of the Great Black-backed but with a slimmer bill and more attenuated look to the rear - a classic Lesser Black-backed type. The heavy "grease" stain (or neck boa) on the lower hind-neck of the Lesser Black-backed is often present in this age group.
This photo was taken in Barnstable County, Massachusetts in mid-July.