The Oxford English Dictionary traces the use of senior in the sense of “an older person” to the 14th century. In contemporary American English, however, senior is generally taken to be a shortening of the more recent senior citizen, and those who object to the compound are apt to object to the shortened form as well. As with the compound, though, you may have difficulty finding a good alternative. When speaking of older people as a group you can of course use the elderly, but when referring to individuals, as in There were several seniors in the cast, there is no clear alternative besides older people, which you may not feel is a significant improvement.
