National Center for Education Statistics.
Within the U.S. Dept.of Education and with the Institute of Education Sciences this federal site collects and analyzes data related to education. It includes reports, surveys and tables of statistics.

IPEDS (Integrated Postsecondary Education Data System)
Surveys which collect data from providers of postsecondary education including enrollments, program completions, faculty, staff, and finances.

MarketResearch.com Academic 
Type in the word college and obtain reports such as:
"The U.S. College Market, 2001"
"The U.S. Youth Market:How 15-24 Year-Old Consumers...
"The U.S. Youth Market: Deciphering the Diverse Life Stages..."
Keep checking as more recent reports may be added sporadically.

National Association of College Stores  (NACS)
Their Media Room has press releases and some statistics and reports on the college textbook market including demographics and spending by college students.

Webnet Catalog for books such as "Millennials: Americans Born 1977-1994" which contains a section on college students.

"Marketing to the Campus Crowd: How to Capture the $200 Billion College Market, 2004".  Available through ebrary (an electronic book library). Type in the title and read it online.