The Iron Man star rented out the Barker Hangar in Santa Monica, California on 4 April (15) to celebrate his age milestone with celebrity friends like Gwyneth Paltrow, Jason Bateman, Reese Witherspoon, Jennifer Aniston and her fiance Justin Theroux and family members, and he hired two of his favourite bands, Duran Duran and Steely Dan, to perform at the party.

He says, "It was kind of a big deal... I was stoked."

Downey, Jr. reveals he was called up in front of the crowd to belt out a tune with Steely Dan frontman Donald Fagen, but his mind went blank and he had to pretend to sing along, much to his own embarrassment.

He tells U.S. talk show host David Letterman, "They said, 'Come up and sing Reelin' in the Years because we saw you sing that at a show once. Come on up, come on up!', and I forgot the words 'cause I was a little excited to be onstage with Steely Dan. I kinda lip-synced it to nothing!"