When determining the age of a gull, we use the presence or absence of dark markings at specific areas in the plumage and on the bill to distinguish sub-adults from adults.
One of these features is the presence of a dark spot on one or more tertials in sub-adults, typically gulls that are one cycle away from obtaining full adult plumage. The size of these tertial spots varies from small dots with faded edges to large, sharp-edged spots (sometimes referred to as ‘ink spots’).
Because the tertials are hidden in flight, they are best observed in resting birds.
Although this feature is typical for sub-adults in certain species, it is sometimes seen in adults as well (meaning: an individual in full adult plumage without any sub-adult markings other than the dark tertial spots).
However, because we don’t see these features in adults of a known age and older than 3 years (small gulls) or 5 years (large gulls), we usually consider these to be young adults that are in their first year in which they have obtained their adult plumage.
Below follow some examples and exceptions in various species of gulls that I have observed over the last couple of years. As you can see, they are all of light-winged species. (One example exists though of an Atlantic Yellow-Legged Gull, see the link in the references at the end of this post.)
American Herring Gull
When gull watching in the area around Chicago, USA, in February 2014, I was surprised to see how common the presence of dark tertial spots was in sub-adult American Herring Gull Larus smithsonianus.
European Herring Gull
Tertial spots are very rare in European Herring Gull Larus argentatus, and only sometimes occur in 4th-calendar year birds and hardly ever in adults.
Personally I have only came across a few examples.
I saw this individual on 27 April 2014 at the Maasvlakte, a large breeding colony near Rotterdam, the Netherlands:
The dark greater primary coverts are indicative for a 4th-calendar year individual, but the adult looking primaries point more towards a 5th-calendar year bird.
This also seems to apply to the wing coverts: browner on the underwing (4th-calendar year type) and only a slight hue on the upper wing (5th-calendar year type) .
Noteworthy as well are the small black markings on the outer web of P5 and especially P4, as well as the staggered and sharply angled shape of the tongue on the underside of P10.
The tail is completely white apart from a small black spot on the outer left feather, but this is not a good indication of age.
A fifth-calendar year, ringed as Green YBPS in the gull colony of IJmuiden (Forteiland), seen in June 2015 at the same location.
Green B9, ringed as an adult female in the breeding colony of Europoort, near Rotterdam, the Netherlands in May 2015 and seen aged older than 5th calendar year in November 2016 at Katwijk aan Zee, the Netherlands:
It was not reported if this individual also showed this tertial spot in 2015, so it is assumed that it was not present.
Ring-billed Gull and Common Gull
Tertial spots are a common feature in 3rd-calendar year Ring-billed Gulls Larus delawarensis and its European counterpart Common Gull Larus canus.
Tertial spots in Black-headed Gulls Chroicocephalus ridibundus are not uncommon but still pretty rare, especially in adults.
Some examples of sub-adult California Gulls Larus californicus.
Identification of American Herring Gull in a western European context by Pat Lonergan & Killian Mullarney, Dutch Birding 26-1, 2004
Atlantic Islands Yellow-legged Gulls: an identification gallery by Philippe Dubois, BIRDING WORLD, Vol 14-7 (see plate 22)
If you are aware of examples in other species (especially European), I would love to see them.