Master of feedback and flaming grand finales, Jimi Hendrix is now rocking the auction house.

In November, an inscribed copy of the 1967 album Axis: Bold as Love by the Jimi Hendrix Experience, his band, went for $20,000—twice its estimate—at Christie's. His Fender Stratocaster guitar brought $168,000 at the same sale. Fellow rocker Eric Clapton's Strat, known as Blackie, still holds the auction record of nearly $1 million, paid in 2004. But many experts believe the Stratocaster that Hendrix wielded at Woodstock would fetch more.

It's not just musical prowess that is driving these prices, nor the intrinsic value of the guitars, originally purchased for a few hundred dollars. As with art, what's considered hot among collectibles shifts with each generation. People tend to collect out of a sense of nostalgia, often for their own youth. And right now, the market reflects the taste of the generation with the most disposable income: baby boomers.

"In the early '80s, baby boomers started getting nostalgic about the Beatles and the Rolling Stones, and that gave birth to the whole market for pop culture," says Simeon Lipman, Christie's popular-culture specialist. Demand for rock-and-roll relics has been growing ever since, along with comic books and sports memorabilia from the '50s and '60s. Comic-book collecting peaked in the late '80s, but early Iron Man comics, from the '60s, still sell for as much as $6,500 on eBay, and baseballs signed by Lou Gehrig or Babe Ruth can go for $7,000.

But appraisers say it's a mistake to assume that what's popular now will stay that way. Generation X is unlikely to have the same fascination with Eric Clapton or muscle cars like the Corvette and Camaro. "When I started collecting records, opera 78s were in big demand. But now, most of those collectors are dead and opera is no longer hot," says Michael Sherman, a collector and appraiser based in Orange County, California. "Baby boomers have the same nostalgia for rock and roll, and we pay a lot for it. Interests shift."

Prices for choice Beatles memorabilia are still astronomical but starting to drop, experts say, while demand—and prices—for items connected to popular '90s bands such as Nirvana and Guns 'N Roses is rising. "A guitar of Kurt Cobain's was worth maybe $10,000 a decade ago," says Lipman. "Today, you couldn't touch one for less than $100,000. You're seeing younger collectors getting to the point where they have a little expendable income and are beginning to long for their glory days. That's what this is all about. It's an organic kind of madness, the collecting thing."