College is wasted on the young. While the kids moan about their homework, the grown-ups -- leafing wistfully through the catalogues -- dream of sitting in on lectures.

At retirement, many a grown-up finally quits dreaming. Large numbers of older people are resuming their studies, sometimes for degree credit, sometimes just for courses that interest them.

The good news is that, for many seniors, study can be cheap. At least 18 states now waive tuition for older students at state and local colleges and universities. Eligibility begins anywhere from 55 to 65, depending on the state. The student pays only for books, transportation and enrollment fees (sometimes up to $300).

In non-credit programs, often linked to colleges, qualified retirees teach courses to each other. Or you might consider an Elderhostel program, which provide for a short stay on a college campus.

At Pennsylvania State University, it's hard to imagine a more enthusiastic student than Arlene Smith Moore, 66, recently retired from the Butterick Co. in Altoona, Pa., after 17 1/2 years of folding home-sewing patterns. She is now enrolled in Penn State's "Go-60" program, taking tuition-free courses in social work toward an associate degree. When she finishes, she plans to volunteer -- returning to the community her gift of education.

She is not unique. More than a million seniors are enrolled in higher-education degree programs, according to Richard Fischer, associate provost and director of continuing education at the University of Delaware. Seniors who have undergraduate degrees are increasingly going for master's degrees in liberal arts.

If price is a problem, older students are eligible for the same need-based financial aid that is offered to younger students. Another source of money may be your corporation. International Business Machine Corp.'s retirement education assistance plan reimburses $2,500 in tuition expenses for employees and another $2,500 for an employee's spouse. The funds are available from the fifth year before retirement through the third year after.

Growing even faster than the degree programs are academic studies just for the fun of it. These now involve some 130 programs, with 50,000 to 100,000 participants. The seniors themselves decide what will be offered and, in most cases, do the teaching.

For example, the University of North Carolina at Asheville offered a course by Richard Sampson, a retired Foreign Service officer, on "The Politics, Problems and Challenges of Southeast Asia and the Pacific Rim." Frank Marvin Jr., a retired minister with a PhD in history, taught a course on the crusades and medieval religion. Both courses filled up quickly, says executive director Ron Manheimer. So do computer studies. Cost: $80 per semester, no matter how many courses are taken.

The Rochester Institute of Technology's three-year-old Athenaeum program was such a hit that people were lying about their ages to get in, Dean Mark Blazey says. So the age limit was lowered to 50 from 55. Cost: $200 a year, for as many courses as you want, plus use of the athletic facilities. One recent guest lecture: "An Anthropological View of Some Stupid Fallacies About Our Retirement Years." Local senior citizen groups will know of any such schools in your area.

Elderhostel offers a wide choice of programs at some 1,500 colleges and universities in the United States and abroad. The U.S. programs are one-week long with three classes a day (no homework, no grades), at an average cost of $270 for room in a college dormitory, board, instruction and extracurricular activities. You pay transportation. Needy seniors can apply for financial aid.

The programs abroad are more expensive. Two weeks at two Italian universities studying Italian politics, art and culture cost $2,020, including transportation from New York. Three weeks at three different British universities come to $2,234.

There are also study cruises, archaeological digs and home stays with foreign nationals. For a free catalogue, send your name and address on a postcard to Elderhostel, 75 Federal St., Boston, Mass. 02110.