Nothing disturbs certain rock fans more than watching (and hearing) their heroes grow old. The high energy of rebellious youth gives way to the mellowed-out restraint of middle age, and the chagrined followers bellow, “How dare those guys play like they’re 35?”

How dare they? Well, because they aren’t kids anymore. And, if they have any honesty and value as artists, they will make music that chronicles their current passage in life. It’s a topic worth considering as the baby-faced stars of two decades ago slide into their 40s.

PETER TOWNSHEND, the guitarist-composer of the Who and one of rock’s monumental figures, displayed probably more energy onstage and in the studio than any performer of the 1960s and early '70s, with the exception of the Who’s drummer and ace hotel-wrecker, Keith Moon.

Now Townshend is 37, and his writing of late for the Who and for his own solo albums has become increasingly laid-back and introspective.

His new LP, “All the Best Cowboys Have Chinese Eyes” (Atco, SD 38-149), continues that trend. It succeeds as solid, enjoyable music and as a declaration of Townshend’s state of mind. The album glows with his triumph over two years of confusion, alcoholism, drug addiction and separation from his wife and two daughters.

HIS BINGE took him from the rock hangouts of London to the lazy beaches of Southern California and then to London’s high-society clubs. He kept trying on new images, as he writes in “Uniforms,” a song from the new 11-track record.

What resulted was disgust at the trendies (including Townshend) who conformed to whatever happened to be fashionable: “I am frightened, you are frightened. Should we get our trousers tightened?”

“Can’t pretend that growing older never hurts.” —Peter Townshend lyric

He also attacks meaningless lifestyles in “Exquisitely Bored” (“in California”), “Communication,” “Stardom in Action” and “Somebody Saved Me.” In the latter, Townshend himself is that “somebody.” He sings of waking up “clean” from his anguished two years, while “my friend”—the former boozing, confused, thrill-mad Townshend—is “stone dead.”

HIS SAVING grace is love. He realizes that, with his family and the basic goodness inside himself, he has all he needs. He sings in “Slit Skirts,” the album’s shimmering finale, that one doesn’t “have to be drunk to try a new dance ... Why aren’t we thinking up romance? Why can’t we drink it up, true heart romance.”

So forget the image-seeking described in “Uniforms.” In “Stop Hurting People,” he writes, “Love conquers poses. Love smashes stances. Love crushes angles into black.”

OLD WHO fanatics will no doubt decry the lack of explosive energy in the music. The power chords and the frenetic drums are at a bare minimum here.

Townshend is more concerned, as he was on his last solo album, with arrangements that are “tasteful” (his word), choosing to let loose only during sections of a few songs.

Giving first-class support to his vocals, guitars and synthesizers are drummers Mark Brzezicki and Simon Phillips and bassist Tony Butler. Virginia Astley, Peter Hope-Evans, Jody Linscott and Chris Stainton add instrumental help on a few cuts.

As for the album’s strange title, Townshend explained in a magazine interview that “All the best cowboys have Chinese eyes” means that you can’t hide who you are — in other words, the personality of even the most macho cowboy can be discerned in his eyes.

This terrific new LP indicates that Townshend is through hiding who he really is. “Can’t pretend that growing older never hurts,” he sings. Granted, it hurts, but he has decided to overcome it and become that rarity of rarities, a rock musician who ages gracefully.