Age: Both birds appear to have old, faded juvenile primaries. Note the absence of p6/p7, thus they're actively molting primaries, as well as tertials and wing coverts. This "complete molt" is the 2nd prebasic molt and so we can safely say these birds are in their 2nd (molt) cycle.
Identification: A nice size comparison of two species frequently seen together on the Atlantic. The individual on the left has a mostly black bill, contrasting white head and solid, dark upperparts. It also appears slimmer and decidedly smaller. It is a rather standard 2nd cycle Lesser Black-backed. The blocky-headed gull on the right, with nondescript upperparts and relatively large paling bill is "just" a Herring Gull. The retained primaries and tertials have bleached significantly, but this is rather typical for a July bird.