Some say Christina Aguilera's rare mix of old-school talent and new-school sensuality makes her appealing to fans ages 8 to 80. Last weekend, the Grammy-winning chart-topper put that theory to the test.

Aguilera was one of several high-profile celebrities who surprised more than 500 guests with an appearance at the 84th birthday celebration for millionaire lumber magnate Joe Hardy in southwestern Pennsylvania. Joining Robin Williams, Bette Midler and the Broadway cast of "A Chorus Line," Aguilera finished off the 12-hour party in style with a performance of "Beautiful" and what could be her next single — the new classic "Happy Birthday, Mr. Hardy."

According to the Pittsburgh Post-Gazette, guests arriving at the party on Saturday were as unaware of what was in store for them as the guest of honor. Organized by Hardy's children, the party took on a special significance since the patriarch's age hit the same number as the company he began more than 50 years ago: 84 Lumber.

Arriving at the posh Nemacolin Woodlands Resort & Spa in Farmington, Pennsylvania, guests were given a program promising "a light luncheon with an extraordinary celebrity feature entertainer." Soon enough, Midler took the stage with a full orchestra behind her, teasing Hardy about his seven children and ample bank account. Hardy, sharing a table with various politicians and other notables, laughed heartily.

Next up was "Happy Feet" star Williams, the after-dinner performer who similarly teased Hardy during a comedic performance. Afterward, the star shared a quiet moment at the cigar bar with the man of honor while guests danced.

It was the unexpected appearance by Aguilera, however, that truly became the talk of the evening. Appearing amid the lavish flower displays, beef and lobster dinners and announcements that the local airport was being renamed after Hardy, the Pittsburgh-area native serenaded the hard-partying octogenarian, much to the billionaire's delight.

Over the last few years, celebrity appearances at lavish bat mitzvahs and other coming-of-age parties have become increasingly common, featuring such highly paid special guests as Aerosmith, Tom Petty and 50 Cent. Judging by the super-powered bash that Joe Hardy's kids put together this year, Fiddy and friends might want to start thinking up lyrics that rhyme with "85."

Taken from VH1