"Baby boomers" are people who are born between 1946 and 1964. That means as of this writing in 2012, baby boomer are within the age range of 48 to 66.
With average life expectancy of 78.1 years in the United States, and 69.4 year worldwide, middle age is 39 and 35 years old respectively.
Senior citizen is a person at or above retirement age -- which is 65 in the United States.
So the majority of baby boomers are not quite senior citizen. Hence they are middle age.