Olivia Newton-John in the film ‘Grease’

John Travolta (right) and Olivia Newton-John pose in a scene from near the end of the hit 1978 musical Grease. Travolta turned 24, and Newton-John turned 30, the year the film came out, yet they were cast as high schoolers in that way that Hollywood loves. Audiences loved it too -- Grease was the top-grossing film of 1978.