On the coast between Los Angeles and San Francisco, lettuce, Lactuca sativa, seems to grow throughout the year. None of it actually grows in every season. Some varieties merely produce late enough for varieties that produce earliest to replace them. Slightly farther inland, lettuce is really only a cool season vegetable of early spring and autumn. The last new plants should finish by May.
There are many varieties of lettuce. Some are more tolerant of warmth than others. They perform well both late in their early season, and early in their late season. Others are more tolerant of cool weather. They can start early in their early season, and continue late in their late season. No variety produces through the coldest part of winter. Warmth initiates bolting (bloom), which ruins flavor.
The three types of lettuce that are most popular here are leaf, head and romaine. Leaf lettuce is the most variable. It can be blotched, bronzed or reddish, with variably ruffly texture. Some types of leaf lettuce mature in about a month. Some of the more substantial varieties of head and romaine lettuce start early, and can take nearly four months to mature. They can get to a foot wide and tall.