It may be they were always getting older. From the Los Angeles Times:

Like any other panic-inducing assumption, the “graying audience” theory bears examining, much as did the widely quoted — and since disproved — “fact” from a 1986 Newsweek article that a single woman over 35 is less likely to get married than to be attacked by terrorists. (Well, at least the poor thing has tickets to the symphony.)….

True, Iyengar adds, the median age of the general population is creeping up as well: It was 40 in 1982 and had reached 45 by 2002. Still, that average is not increasing as fast as the age of the performing arts audience. “You are not seeing a 1-to-1 ratio,” Iyengar says. “Even in jazz, that typically has the lowest median audience age of all the art forms — in 1982, the median age was 29, and in 2002 it was 43.”

Even so, representatives of such organizations also offer compelling reasons why seeing gray hair — or, at least, gray roots — in the audience is (a) nothing new and (b) not necessarily a cause for panic, because, at least so far, there has always been “new gray” waiting in the wings to replace the old.

“A colleague of mine says the audience isn’t graying — it’s always been gray,” says Teresa Eyring, executive director of Theatre Communications Group, a national service organization for American nonprofit theaters.