One in four American college students is over 30 years old, part of a graying student population that is offsetting a sharp drop in the number of traditional-age college students, according to a Census Bureau report released yesterday.

Of the 13.2 million students enrolled at the nation's colleges and universities in 1989, about 40 percent were age 25 or older. The traditional 18-to-24-year-old group comprises about 60 percent of the student population. In the '70s, that group made up 75 percent.

About 3.3 million college students were aged 30 or older, double the number 15 years earlier.