At age 26, Christina Aguilera is among the top-grossing artists in the world for the first half of 2007. Conversely, the plug was recently pulled on 25-year-old Kelly Clarkson's summer arena tour because of lower-than-expected ticket sales.

The varied fortunes of these two promising pop artists aptly illustrates the fragility of box-office stardom—and why the concert industry is grappling with a serious artist-development conundrum. Even as touring remains immune to many digitally induced music business ills, the concert business will ultimately face a disconcerting changing of the guard that must be reckoned with.

Fact one: Six of the top 10 and 11 of the top 25 tours of 2006 feature acts that will qualify for their AARP cards within 10 years.

Fact two: Of the remaining acts, four—Tim McGraw, Kenny Chesney, Pearl Jam and Dave Matthews Band—have shown the kind of consistency during the past 10 years that indicates they will remain top draws a decade from now. And, to paraphrase a stockbroker's cover-your-ass mantra, past performance is no assurance of future results.

Among the 10 top-grossing acts of the last decade, only one -- Dave Matthews Band -- broke in the 1990s or later. The rest first cut their touring teeth in the 1970s or earlier. In fact, outside of DMB the "newest" band in the bunch is U2.

While the Rolling Stones have blazed a trail that shows U2 could well have another 20 years of hard touring ahead of it, simple biology tells us that the majority of the top earners of the past decade probably should not be counted on to deliver box-office gold in 2020.

Read the full article here -- including the best bets on who will drive the industry in 2020, and thoughts from the business' heavy-weights, including top executives at AEG Live and Live Nation.