"Age is mind over matter," cracked Muhammad AM last fall, before being whupped by a 28-year-old. "As long as you don't mind, it don't matter."

But a 40th birthday—that milestone which marks middle age—matters a great deal, especially perhaps to the rebellious '60s generation. Its slogan, after all, was "Don't trust anyone over 30." The decades have slipped by and a lot of erstwhile flower children find themselves, as Jethro Tull sang, "Too old to rock 'n' roll, too young to die."

For Tammy Wynette, turning 40 did not seem all that catastrophic until daughter Georgette, 10, let slip that her teacher was ancient—all of 26. The singer, who admittedly had been feeling her years after three major stomach operations in 1981, has since adjusted her thinking. As Tammy puts it, "Now when I'm reaching the end of my rope, I just tie a knot and hang on."

In gentler times it was defiantly said that life begins at 40 (the title of a 1933 best-seller). That is still the case for some, even possibly Barbra Streisand. Twelve years ago she bought the film rights to an Isaac Bashevis Singer story, Yentl, the Yeshiva Boy, about a Jewish girl who disguises herself as a male to enter a religious school. Studios were unenthusiastic. Finally Barbra sold the package to MGM-United Artists, and she will both star and direct. But there is one problem. Now the actress, 40 in April, must play a rabbinical student half her age.