LOS ANGELES — “The Great Gatsby,” with Leonardo DiCaprio, scored a $51.1 million debut that made it a surprisingly strong runner-up behind “Iron Man 3,” which made $72.5 million.

Estimates yesterday put “Gatsby” far above forecasts of $35 million to $40 million.

It was a rare box-office smash for a film aimed at adults during the youth-minded summer season. Viewers over 25 made up 69 percent of the film’s audience, Warner Bros. said.