Age: The outer primaries on both individuals are tattered and faded, but the upperparts are largely new. There are obvious molt gaps throughout the upperwing coverts, especially the greater coverts. Both birds appear to be in their 2nd molt cycle with retained juvenile primaries
Identification: The size difference seen here is real, as can be seen from the bill and head dimensions, as well as body and leg girth. The individual on the left is a standard 2nd cycle Great Black-backed Gull with white body, checkered upperparts and a stout bill. On the right, we see a thin bill, sleek rear to the body, and what looks like a svelte, long-winged gull. The upperparts are too solidly dark and checkered for a 2nd cycle Herring, and the clean white underparts also don't evoke thoughts of that species. This bird is a Lesser Black-backed with a somewhat cleaner white head and neck than average, and upperparts that still don't show any signs of adult-like gray. The bicolored bill is also more advanced than many similar-aged Lessers which typically have more black blended through the bill base. Here's my idea of a more average-looking LBBG for comparison.
Of interest here is that GBBG and LBBG don't often look this similar at one year of age, but these two are surprisingly similar. Size and structure are very helpful here.