Programs, books and videocassettes are available for those who want help becoming -- and staying -- fit after 50. Among them: The Dance Exchange offers dance classes for people over 60 and has a performing troupe including senior dancers. For information, 229-8036. Low-Stress Exercise, particularly helpful for older people and those with special physical concerns, offered by exercise therapist Asta O'Donnell Inc. 815 Royal St., Alexandria, Va. 22314; 836-3070. Adults Health and Development Program at the University of Maryland matches college students with participants. The program includes inter-generational exercise, nutrition lectures, social time and discussions. Call Dr. Dan Leviton, 431-3733. "Exercise for Older Americans," a free resource guide, is available from the National Health Information Clearinghouse, P.O. Box 113, Washington, D.C. 20013-1133. "Low-Stress Fitness," by Millie Brown, The Body Press, a division of HPBooks Inc., 1985; $8.95. "Fitness After Fifty," by Herbert A. deVries, Scribner, 1982; $5.95. "Fitness For Life," by Theodore Berland, published by American Association of Retired Persons, 1986. Send $12.95 plus $1 for postage to Farrar, Straus Giroux, 19 Union Square West, New York, N.Y. 10003. "Good Moves -- for a Lifetime of Good Health," by Asta O'Donnell, a 90-minute tape containing two 45-minute stretching and strengthening exercise programs is available for $74.95 (includes shipping and handling) from Rudisill Inc., 815 N. Royal St., Alexandria, Va. 22314. "Jane Fonda's Prime-Time Workout," Karl Lorimer, $39.95. "Do It Debbie's Way," with actress Debbie Reynolds, Video Associates, Inc. $39.95.