F. Scott Fitgerald said that there were no second acts in American lives. Too bad he’s not around today to check out Robert Downey Jr.

After a record-breaking descent into drugs, bizarre behavior, jail and career destruction, Downey has staged an amazing Phoenix-like rebirth. He garnered good reviews in “Zodiac” but it’s hardly the kind of role that marks his newest effort. How many actors get a breakout role at 43?

That’s just what Downey’s performance in “Iron Man” is. At middle-age, he’s a comic book super hero and one of the most interesting ones to date. In this John

Favreau fantasy based on a Marvel Comics character, Downey plays Tony Stark. Tony’s a high-living, engineering genius who heads a giant arms manufacturing company. War is good business and Tony doesn’t let anything like a conscience get in the way of his sybaritic life style.

That is, until his pleasuring-seeking world is rocked on a promotional trip to Afghanistan. His convoy is attacked and he comes out of the fray with life-threatening shrapnel wounds and captured by a bunch of terrorists who want him to build them a state-of-the-art missile system. Instead, he and a fellow prisoner devise an invulnerable suit of armor that’s capable of plenty of mass destruction of its own.

Tony’s Middle Eastern adventure gives him an epiphany. He’s seen all the human misery his products cause and he wants to stop it. So he builds a new and improved suit of armor that allows him to fly and blow up anything in his way.

This is Downey’s movie but he gets plenty of help. A radiant Gwyneth Paltrow plays Pepper Potts, his loyal assistant, and she gets to show her bonafide babe side.

Look for the chemistry to heat up in the next installment — and there’s surely going to be a next installment.

Terrence Howard plays Tony’s military buddy and baby sitter. Jeff Bridges is the business wing man with a sinister side.

Favreau allows plenty of smart banter and knowing asides amid the hardware but he doesn’t stint on action. This looks like the warm weather blockbuster to beat and it propels Downey to a whole new level of stardom.