Rihanna Surprise Guest of McCartney at 'Oldchella'

The Desert Trip festival is all about showcasing rock veterans and their baby-boomer fans but its second weekend featured one of the top stars of the young generation -- Rihanna.

The R&B superstar joined Paul McCartney on stage Saturday night in the California desert to join in "FourFiveSeconds," the song the former Beatle released last year with both Rihanna and rap giant Kanye West.

"We finally found someone under 50," McCartney said as he invited the 28-year-old Rihanna to the stage, according to postings on social media.

The inaugural Desert Trip is taking place over two consecutive weekends with identical lineups made up of rock legends who have never played together before.

Besides McCartney, the festival features Bob Dylan -- who in between the two weekends won the Nobel Prize for Literature -- as well as the Rolling Stones, Neil Young, The Who and Roger Waters.

The festival was put together by the promoters of Coachella, which takes place each April at the same site and is famous for surprise guests showing up on stage.

With its all-star line-up and premium prices on seats and amenities, Desert Trip -- nicknamed "Oldchella" by both detractors and fans -- is expected to be the highest-grossing festival of all time.