"The trouble with earthlings," declares anthropologist Ashley Montagu--as if peering down from a mythical spaceship--"is their early adulthood. As long as they are young, they are lovable, open-hearted, tolerant, eager to learn and to collaborate. They can even be induced to play with one another.

"Most adults are mortal enemies. So the only educational problem earth has is how to keep humans young. The ideal should be to prolong childhood up to 60 years."

After a lifetime devoted to "understanding the nature of human nature" the 77-year-old Princeton professor and author of more than 40 books--including The Nature of Human Aggression, The Elephant Man and The Natural Superiority of Women--sums up the secret of life: "Die young as late as possible."

Individual happiness and "humanity in general" is jeopardized, maintains Montagu, by "our tendency to believe 'grown-ups' and 'children' are two separate classes of beings. Because adults possess the power and the strength, we spend the years of our childhood yearning to be grown up . . .

"Yet the truth about the human species is that in body, spirit, feeling and conduct we are designed to grow and develop in ways that emphasize, rather than minimize, childlike traits such as imagination, creativity, flexibility, playfulness. We were never intended to grow up into the kind of adults most of us have become."

Most adults, he says, have grown a shell around the "pitiful store of knowledge and wisdom learned through formal schooling." From graduation on, "they resist with enormous energy any attempts to pierce that shell with anything new."

To counter this widespread "hardening of the mind," Montagu is campaigning for "a whole new way of thinking about human beings"--a philosophy he explores in his new book, Growing Young (McGraw-Hill, 303 pages, $12.95). "Instead of idealizing youth and dreading old age," he says, "human beings must be encouraged to develop all their lives. You're killing yourself if you continue to act like an adult."

Scientists call the process of "growing young" neoteny or paedomorphosis, both of which mean, says Montagu, "the retention into adult life of those human traits associated with childhood, with fetuses and even with the juvenile and fetal traits of our primitive ancestors.

"It is a process that has played a fundamental role in the evolution of the human species and in the development of every human being who has ever lived. (It) is one of the major qualities that differentiates human beings from other animals."

Montagu, perhaps his own best example, is both dapper and distinguished, warm-mannered and curious. "I enjoy new things . . . I dine with students three times a week . . . I write to people whose books I read . . . I've had a 9-year correspondence with a troubled young woman who read my book and wrote me . . . The greatest pleasure in life comes from giving to people."

And he dances daily. "I'm a child of the Jazz Age, so I put on some Big Band music. One thing I am proudest of is that someone once compared me to Fred Astaire."

Montagu first began thinking about "growing young, back in my childhood, in London. When adults wanted to show affection they would grasp my cheek between their thumb and forefinger, tweak it and say 'Good boy!'

"This left me with the insoluble equation, 'If I'm such a nice child, why do they inflict such pain upon me?' Since that time I became bent on discovering how people behave and what causes them to behave that way."

At the University of London he was dissuaded from becoming a psychiatrist ("They called them 'alienists' then") and encouraged to study anthropology. "It was perfect for my interests because it enabled me to see the interrelatedness of things.

"What I've been having all my life," he says in a conspiratorial whisper, "is a good time. The trick in life is to find something one enjoys doing and make that one's way of earning a living.

"Just because you're advanced in years doesn't mean you have to act like some old dodo. It's never too late to get out of your rocking chair and get into rock and roll."