Aging this one is a bit trickier than usual, and should be done in conjunction with its identity. The underbody and wing linings are very adult-like, but confusing here is a largely black tail, no mirrors and a pale iris.
This species has a longer and thinner than average bill when compared to most other 4 years gulls. The upperparts, at least what we can detect from them on the upperside of the far wing, are somewhere between a Laughing Gull and pale-end Lesser Black-backed Gull.
The yellow legs and proximal-red, distal-black, bill tip pattern are very helpful to narrowing this down to Black-tailed Gull. The black tailband and uniformly broad white tips to the tail is key. No other species in North America will show this.
As for its age, the black splotches across the secondary centers and dusky under primary coverts make it fairly safe to call it a 3rd cycle type.
A complete dorsal view of this beauty:
|3rd Cycle Black-tailed Gull. Choshi, Japan. December.|