Dancers age, of course, just like everyone else. But because they spend their professional lives cheating forces of nature like gravity, metabolism and muscular inertia out of their inevitable due, dancers are much better than the rest of us at hiding or deferring the passage of time.

A perfect example is Alicia Alonso, ballerina extraordinary, who is here dancing with the Ballet Nacional de Cuba in the company's United States debut at the Kennedy Center, over the next two weeks.

At 56, a grandmother, Alonso is still taking daily class, attending rehearsals and performing regularly, more than a decade beyond the time when most female ballet dancers begin to think of hanging up their toe shoes once and for all. Wearing a pink, one-piece slack suit, her face flushed with color, she's as slim and erect as a popular, and hse walks into the room for the interview with a springy dancer's stride, a mark of her trade. At her side is her husband, trim and distinguished looking Pedro Simon, a lawyer who's also a dance critic and editor of Cuba's first ballet periodical.

When she talks, Alonso's hands and arms dart into the air, scooping out isual similes, exclamations and italics. "How do I feel about being back in this country again? I am very, very excited," she says, "particularly to be here now with the Ballet Nacional de Cuba for the very first time."

Alonso herself spent half her life in this country, first as a young ballet student in New York, then dancing in musicals and with such companies as Ballet Caravan, Ballet Theatre and the Ballet Russe de Monte Carlo. But all the while she was becoming one of the elite personnages of the ballet world through her superb dancing and dramatic portrayals here and abroad, she was also nurturing her dream of establishing a strong native company in Cuba.

With the project well under way at the time of the Cuban revolution, she returned to her homeland once again to tend the growing institution. This time, however, political strains being what they were, she was unable to revisit this country for 15 years, an absence finally ended in 1975 by her reappearance with American Ballet Theatre.

Coming back with her own company, though, is like an entirely new beginning for Alonso.

"You know," she says, "we started this company in 1948 - that's a long time ago - and we did it with the help of a great many fine dancers from America, from both North and South America. From the United States, for example, we had Igor Youskevitch and Royes Fernandez, with whom I danced so often at Ballet Theatre performances, and Melissa Hayden, and many others. I imagine now these dancers are going to have a strange mixture of feelings, when they see how much has happened to the company they remember from its starting days. I think it's going to be a very stange experience for them - and a very beautiful one."

By now the Ballet Nacional de Cuba numbers over 110 dancers - that's appreciably more than our largest company, the New York City Ballet, and in a country whose total population is about 9 1/2 million. Some 85 dancers will be performing at the Kennedy Center; the rest are back in havana, performing as a separate unit. Cuba also boasts a second company, the Ballet Camaguey, led by Alonso's former husband, Fernando Alonso.

Such is the prestige of the BNC in Cuba, though, that Alonso was recently appointed directress of all performing arts activities at Havana's main hall, the Garcia Lorca Theater - activities including grand and lyric opera, and orchestral concerts, as well as ballet.

Does Alonso welcome such administrative responsibilities? "Well, yes, I do," she says with a somewhat dubious smile, "but I'm happy to say that I have a lot of good people helping me."

The relatively youthful BNC has already developed a personality of its own. "Certain classic of the repertoire," Alonso notes, "are more or less the same, of course, in every company - the choreography is the same. But even in the classics, there are those 'forgotten' passages that we do in our own way, and the mise-en-scene is very much our own creation.

"For one thing, we are very fussy about staging. Every action, no matter how small, must have a meaning that projects itself clearly to the audience. And our staging tends to be very rich in movement - we try to make everyone on stage participate in the movement, in the dramatic composition."

The Kennedy Center repertoire includes not only such classics as last night's "Giselle," and the "Coppelia" and "Les Sylphides" to come, but also a large number of ballets by Cuban choreographers whose work has never before been seen here.

"In the Cuban repertory," Alsonso says, "we make a conscious effort to introduce Cuban music and folklore through the language of the ballet. One of the distinctive features is the sense of humor tat crops up again and again - it is curious how much this trait has been developed in our culture. Somehow it is very much in our character to be merry and vivacious, and this comes forward in our choreography - and in our dancing, too."

One of the Cuban ballets is a work by Alonso herself. Called "Genesis," it's based on the daring theme of biological conception, and it uses an electronic score by Italian avant-grade composer Luigi Nono. "When I told him about my idea for the ballet," Alonso recalls, "he was very enthusiastic, and said I could use it any way I wanted, cut it any way I pleased. I did cut certain parts - so much of it was exactly right for my idea, but not all of it."

Using 45 dancers and a lead couple (Jorge Esquivel and Josefina Mendez), the ballet symbolically depicts the fertilization of an ovum within a mother's body and the growth of the fetus, culminating in a birth. The orginal, abstract decor by Cuban artist Jesus Soto is like a giant womb enclosing the whole state, within which the dance action proceeds.

"I was very much struck by the fact that everything an expectant mother is feeling is reflected onto the embryo, and I hoped to actually represent this in the choreography," Alonso says. "It is all very scientific - I had many conversations with doctors while I was working on this."

But her previous marriage, Alonso has a daughter Laura, who danced for a time with the company and is now teaching ballet; she also has a grandson, Ivan, who is a member of the corps de ballet of the Ballet Nacional. Asked how old her grandchild is now, she flashes her most archly feminine look and replies, "old enough to be in the ballet."

Looking to the future, Alonso is aflutter with anticipation about an enormously ambitious three-week ballet festival that is scheduled to take place in Havana under her direction starting in late October.

"The idea," she says, "is to stimulate choreographers all around the world, to give them a creative push. We have invited both dancers and choreographers from Italy, France, the United States, Japan, South America, Mexico, the Soviet Union, Denmark. Jerome Robbins will be comings; Alvin Ailey, Glen Tetley, Gerald Arpino, among many others - they'll all contribute new works. I think it is the first time anything like this has been attempted anywhere on this scale."

The "ballet explosion," it seems, has had major reverberations in Cuba. Alonso says they cannot turn out dancers fast enough, so great is the demand - on stage, in the opera, on TV.

However, "The Turning Point," the ballet movie that has all Alonso's old colleagues from ABT in it, has not yet played the Cuban cinemas yet. It did have a private showing one night, but Alonso couldn't make it - because she was performing.

"Ooh, I was so mad," she says. "But I'm determined to get to see it yet."