The American College Theater Festival, which has given theater experience to nearly 100,000 young people, begins a two-week stand tomorrow at the Kennedy Center. As it enters its 10th edition, the festival truly can be said to have come of age.

Its awards have stimulated many new professional careers. Though most of the 100,000 participants will not become professionals, they will have had working experiences that core of future audience, those audiences without which theater can't exist.

The festival began in 1969 as a tentative link between 182 campus theaters. This year it involves 431, some distinctly modest with their makeshift facilities but many with performing arts complexes that are better equipped than additional commercial theaters.

Above all, there is a gathering tie between the professional forces and these hopeful collegiate tyros. The professional world not only is watching and listening, it has become an active particpant.

The sense of coming-of-age is heightened by the death, at 86, of actress Peggy Wood, the star of several careers whose vision began it all.

Daughter of a newspaperman and widow of the World War I poet, John V. A. Weaver. Peggy Wood studied opera with Emma Calve, starred in operettas of Victor Herbert and was the first to sing "I'll See You Again" in Noel Coward's "Bitter Sweet." She turned to Shakespeare and Restoration comedy and spent four years as Ruth in Coward's "Blithe Spirit," taking that ingenious comedy to European troops. For eight years she was the first TV series star, playing Kathryn Forbes' "I Remember Mama" until 1957. She got an Oscar nomination for "The Sound of Music," headed the American national Theater and Academy (ANTA) and helped quicken the State Department's cultural exchange program.

So, when this formidable, voluble verteran sailed into her idea nearly 20 years ago, I had to listen. She'd been playing Electra, a role Broadway has denied almost everyone, with a college theater group, and with another group she had done her "Mama" role.

"What's going on in the colleges you will not believe!" she exulted. "Such talents, such interest in the arts and crafts of theater. America must be made aware of our glorious young!"

Peggy Wood didn't let up until she'd corralled the Smithsonian's S. Dillon Ripley. The Kennedy Center's Roger L. Stevens grasped her idea that a national college alliance fitted naturally in the goals Congress set for the Center.

With ANTA as her base, she involved the American Theater Association. She caught corporate sponsors in her web - American Airlines was the first and Amoco was the most faithful.

The organisation kept growing. A phase ended two weeks before Peggy Wood's death with the death also of director-manager Frank Cassidy, who served six trend-setting years as the festival's executive producer. Under Cassidy, programs for playwrights and Irene Ryan's scholarships for performers were introduced.

This year's festival begins with three performances of "Sideshow," the first work to win both the playwriting and the David Library awards. It is a musical about former vice president John Nance Garner by Rick Smith of Angelo State University, appropriately enough in Garner's home state of Texas.

Among the fortnight's other novelties will be the participation of Smith College, the first women's college to be represented. Smith has chosen Moliere's women's lib comedy. "The Learned Ladies" as its entry for April 21-22.

An always intriguing highlight comes a week from tonight, Irene Ryan Night, when 13 regional winners will compete in "An Evening of Scenes" for scholarships funded by TV's "Beverly Hillbillies" Granny, who regretted that she'd not had the opportunity for formal education.

Past prize-winners in the festival have been making career headway. Lynn Topping, a co-winner last year, is under contract to Univeral Pictures and has appeared in three series and a film, "Aspen." After winning the Ryan in '76, Kathy Monteleone studied in London, played Maggie in "Cat on a Hot Tin Roof" and appeared with a mime troupe in France.

Judi Ann Mason, who won both the McDonald chain's Lorraine Hansberry and the Norman Lear writing awards, is on Lear's "Good Times" staff and has a black "Children's Musical for Adults" cooking. Richard Harlan, the '76 Lear winner, contributed a recent "Maude" episode, and his eight plays for "The Robin Hood Players" has resulted in thousands of performances by touring groups. Michael Moye, of last year's "Tilt," writes for "Good Times."

These productions and scholarship contestants get to Washington through an organizational maze. The nation is divided into 13 regions. Every autumn armies of judges cover college productions for the mid-winter regional festivals. From these, seven plays are chosen for the April Washington festival. For the colleges the challenge is to keep one production alive an entire school year. For the judges, the challenge is to choose not necessarily the best, but a representative selection.

Meshing the participating groups - the Kennedy Center, the Alliance for Arts Education, the Smithsonian Institution, the American Theater Association and the vitally important continuing sponsor, Amoco - is a lesson in Roger Stevens' organizational genius.

Stevens hopes that the Center's new rooftop theater, a Bicentennial gift fromf Japan, will be ready for next spring's festival, for which executive producer Young already has lined up new contests, awards and participants.