Robert Downey Jr. said he started smoking weed at age 8 and did blow with Jack Nicholson and talked about life in jail and his plans for growing old in a revealing interview with Rolling Stone.

The Academy Award nominee divulged that his father started giving him drugs when he was just 8-years-old, which eventually led to a debilitating heroin addiction.

"This stuff, this Mexican sludge, just grabbed you by the f--king heartstrings and tore me apart," he told Rolling Stone. "All those years of snorting coke, and then I accidentally get involved in heroin after smoking crack for the first time. It finally tied my shoelaces together. Smoking dope and smoking coke, you are rendered defenseless. The only way out of that hopeful state is intervention."

Downey Jr. was in and out of jail and rehab from 1996 through 2001 during which time he told a judge "it's like I have a loaded gun in my mouth and my finger's on the trigger, and I like the taste of gunmetal." He tells his interviewer Walter Kirn (author of Up in the Air) about returning home from jail and desperately searching for "that great coke" he'd hidden somewhere in the house.

"And here I was needing to anesthetize like never before," he said. "The wife has moved out, the kid's gone, my life is a f--king babyshambles, and I suddenly make the neuropathic connection that there's nowhere the coke can be but the garbage, and I f--king dig in the thing and there it is, and it's so f--king pure and so clean and there I am, in my own kitchen, cooking up some rock — no Vicodin, no Valium, nothing to take the edge off, barely a trace of fucking Absolut Citron left in the fridge, and I just go, 'This is as good as it gets right now.'"

Downey Jr. has now been sober for seven years, and says he vows to stay that way. The actor, whose newest movie, Iron Man 2, is kicking off the summer movie season next week, says he's looking forward to growing old with his wife Susan Downey.

"Now I think, 'Oh, my God, me and the missus will be [in our L.A. home] until the grandkids attend our funerals.' We'll always be here. We'll never f--ing move from here. Crazy."