Running a successful small business today is for the swift and, more and more, for the young.

Consider Gary Goralnick. During his senior year in high school at Woodland Hills, Calif., he developed and sold an orthodontic retainer cleaning product called "TeenClean." Last year, he sold his product to a major corporation for "a goodly six-figure sum." Goralnick is 19.Lisa Renshaw five years ago took over a bankrupt parking garage near the Amtrak train station in Baltimore. Today, she controls a group of parking lots. Her company grossed $750,000 last year. She is 26.

Ten years ago, when she was 20, Debbi Fields opened a retail cookie store in Palo Alto, Calif. Not a single customer showed up on opening day, so Ms. Fields gave away her cookies. But she persisted. And today she and husband Randy run Mrs. Fields Cookies, sold at 425 stores in 25 states and foreign countries. Revenues last year for the Park City, Utah-based company topped $80 million.

Michael Dell, as a freshman at the University of Texas, used $1,000 of his own money to begin selling and installing IBM computers recycled with cheaper components. In its first month, Dell Computer Corp. took in $180,000. Today the company has 300 employees and annual earnings of about $160 million. Dell is 23.

Dell's success (he really started his business career at age 12, after getting a tax permit to sell baseball cards) was good enough to win him top honors from the Association of Collegiate Entrepreneurs, which came up with a list of the 100 top under-30 small business owners in the country. The honorees were ranked by 1987 earnings.

Second on the list was Randy Miller, who at 17 (he's 23 today) revived the family's water seltzer business. Today, Original New York Seltzer is sold across the country and racked up earnings of $100 million. Miller is company president, a millionaire and owns an 80-acre compound in Southern California where he breeds exotic cats (including tigers and cougars).

Tied for 100th place in the ACE rankings were Doug Burgum, head of Great Plains Software in Fargo, N.D., and John Bryant of John Bryant Associates of Beverly Hills, Calif. Each of their firms reported 1987 earnings of about $1 million. Both are 22.

Twenty six of the 100 top young small business owners were 25 or younger.

All across the country, it seems, more and more youngsters are going into business, and succeeding. Young entrepreneurship isn't all new, of course. Thomas A. Edison went into business when he was a teen-ager. For years, the Junior Achievement Group, based in Colorado Springs, Colo., and the Distributive Education Clubs of America have helped teen-agers set up school classroom businesses.

But only in recent years have teenagers and other young men and women stepped up in large numbers from the classroom business make-believe and started for-real enterprises.

Two outstanding young entrepreneur success stories have been recorded in recent years by Steve Jobs, who founded Apple Computer, and Bill Gates, who founded Microsoft Corp. Apple and Microsoft are among the biggest names in the computer industry.

According to Doug Mellinger, national director of ACE, under-30 entrepreneurs last year generated more than $2.5 billion in revenues. Mellinger, 23, came to ACE after running several small businesses while a student at Syracuse University.

ACE was started in 1983, when entrepreneurship courses were being offered at only a few colleges and universities, including Harvard, Massachusetts Institute of Technology and Wichita State. AEC, which serves mainly as a networking group for under-30 business-minded collegians, is headquartered in Wichita, Kan. ACE has chapters on 250 campuses, works with potential business owners on 50 other campuses and, according to Mellinger, has 20,000 supporters and activists.

AEC also has chapters in Canada, Britain and France and, Mellinger says, Soviet and Chinese representatives have voiced interest in the association.

Most of the young entrepreneurs, according to Mellinger, concede there are tradeoffs to starting a business so early in life and thus setting aside "the joys and freedom of youth." Mellinger says a small business can become a young entrepreneur's "kickball," a reference to the childhood playground game.

"Our members work long hours because they love to," Mellinger says, "not because they have to. Many young business owners become multimillionaires before they're 30. Most don't start a business for money, though, they do it for a challenge. It's not unusual for them to start a number of different businesses.

"They have to. It's addictive."