Robert Downey Jr wishes he'd had ''proper discipline'' in his life at 18.

The 48-year-old actor has admitted using drugs from a young age, emulating his father, Robert Downey. Sr - who was a frequent drug user - but feels his life would have been easier if his dad had put firmer rules in place when he was a youngster.

Speaking to The Sun newspaper, he said: ''I wish I'd had proper discipline in my life at 18.''

The star credits his second wife, Susan Downey - whom he married in 2005 - for getting his life back on track and encouraging him to create order and future plans.

He explained: ''When I finally had some [discipline], from my wife Susan, I realised how much easier things could have been.''

Robert - who claims he started using marijuana at the age of six - has been drug free since 2003 but admits his addiction to substances meant his career took a back seat and he developed a negative attitude towards acting for some time.

He said: ''People who understand drug abuse know that you are not dealing with a rational mind.

''My attitude was always, 'F**k it, I don't care. I don't care about fame. I don't care if I f**k up my career. It's not important to me.''