The Beastie Boys made the world safe for white rappers and they did it a long time ago, before Eminem was old enough to drink, at least legally. This ranks among the most improbable achievements in modern music: We're talking about the mooks who exploded like a shaken beer can in 1986 with the brain-dead frat-rock anthem "(You've Got to) Fight for Your Right (to Party!)" They seemed to have "one-hit wonder" taped on the seat of their pants like a kick-me sign. No one had them pegged for innovators.

That was before "Paul's Boutique" and "Ill Communication" and a handful of other albums, all the work of class clowns who miraculously aced their finals. Through the '90s they never lost their prankish sense of humor and for a long time betrayed little sign of maturity, thank God. But they crafted a beat-savvy style of rap that never seemed like carpetbaggery, and by playing real live instruments, at least once in a while, they discovered some original ways to scale the wall that separated hip-hop and rock. They traded verses with the finest rappers in the world, who lined up for guest vocal spots. Actual street cred flowed their way.

Now the burden of pioneerdom is back in their collective laps. With "To the 5 Boroughs," the Beasties' first album in six years, these three stooges are venturing where no white rappers have been before -- into middle age. It's tricky for any pop-music act to grow old, but rap is harder than most because its main lyrical theme, at least in the non-gangsta categories that the B Boys embrace, is one-upmanship. Basically, your rhymes stink, mine rule. Manic boasting wears well enough on a 28-year-old, but how long can you brag about your skills with a microphone before it starts seeming . . . what's the word? Silly?

"To the 5 Boroughs," to its credit, doesn't ignore the question. The Beasties -- Adam Yauch, Adam Horovitz and Mike Diamond -- make no apologies for their advancing years, nor do they feign youth. Instead, for roughly half the album, they get sharply political, pushing an antiwar, anti-Bush platform in some places and arguing more generally for peace and unity in others. The rest of the time they boast, just like in the old days. And their boasting, as it happens, is all about the old days. Most of their culture riffing will soar right over the heads of anyone under 30.

How many kids, for instance, will grasp the origins of "I've got billions and billions of rhymes to flex / Cause I've more rhymes than Carl Sagan's got turtlenecks"? And how many will understand the meaning of "On a track so slick it'll make you feel all queasy / Make you do like Fred Sanford with 'I'm coming, Weezy!' "? (And how many will spot the blooper: Weezy was the wife in "The Jeffersons," not to be confused with Fred Sanford's deceased wife, Elizabeth, on "Sanford and Son.")

There are plenty of allusions to vintage entertainment: the movie "Being There," the original "Star Trek" series, "Diff'rent Strokes." Even some relatively ancient commercials get name-checked. On "Rhyme the Rhyme Well" everyone hushes after this line: "Shhhhh. You heard me like I'm E.F. Hutton."

Even the music is, in some ways, unashamedly dated. There are apparently no live instruments on the album, just programmed beats, turntable scratches and slices and dices of other music. Among the source material sampled here is "Rapper's Delight," which came out in 1979, and for a tune called "The Brouhaha" there's a lift, believe it or not, from the Partridge Family.

Much of this will elude anyone who started watching TV after Bob Barker turned gray, but it's the best part of "To the 5." It's funny, and though the Boys have snatched some ideas from of-the-moment producers like Fatboy Slim and the Neptunes, most of the music has an old-school familiarity about it.

The political half of "To the 5" is spottier. Adam Yauch, whose voice is the only one of the three that sounds notably worn, gets too specific when he scores Bush for ignoring the Kyoto treaty on "Time to Build." The right balance is struck in the first single, "Ch-Check It Out," which in the tradition of Marvin Gaye's "What's Going On" weaves its message so neatly into the music that the two elements seem part of an organic whole. "Ch-Check" is one of the few protest songs that you could play at a chugging contest.

Nobody really wants a civics lecture from a rap act, and fortunately the Beasties are smart enough to realize that they can't pound the podium unless they're willing to make some goofy mischief, too. At its best, "To the 5 Boroughs" finds a seam: a place where concern about the fate of the planet never distracts too long from the fight. For your right. To par-taay!

The Beastie Boys will appear tomorrow night at the 9:30 club. David Segal will chat online about pop music today at noon on washingtonpost.com.

The Beastie Boys, approaching middle age in their own way: From left, Mike Diamond, Adam Horovitz and Adam Yauch.