Seagulls live anywhere between 5 to 15 years. The average is probably 7 to 10 years. Some have been known to live over 25 years.
Seagulls can drink both fresh and salt water. Most animals are unable to do this, but seagulls have a special pair of glands right above their eyes, specifically designed to flush the salt from their systems through openings in the bill. That is why you'll see seagulls near large bodies of water such as the Great Salk Lake where fresh water may be hard to find.
Some gulls do migrate, especially ones in the northern parts of the world, such as Alaska. Like other migratory birds, they go south during the winter to where the pickins' are better. The migratory patterns vary widely depending on where they reside in the summer, but they can travel thousands of miles to find better food sources.
Source: All About Seagulls