Age/Molt stage: This appears to be a large 3 or 4-year, large white-headed gull. Given the black markings on the tail, we can be sure it's not a definitive adult. It appears the outer primary is still growing in, as are some inner secondaries, and so the prebasic molt hasn't concluded.
Identification: This month's quiz bird is identifiable without seeing the entire head. The black-to-red pattern on the straight greenish bill and noticeable red gape, are perhaps diagnostic for one of our more common western North American species. The upperparts appear to be a darker gray (paler than a Herring or Ring-billed, although admittedly this may be mostly due to lighting and camera settings). Zooming in on the legs, we notice it's not a pink-legged species. Simple! This is a sub-adult California Gull.
Here's a better view of the face.
Imperial County, California. September.