Lance Armstrong admits to first doping 'probably at 21'

Disgraced former American cyclist Lance Armstrong has revealed he first started doping from the age of 21, during his first season as a professional.

"Wow, straight to the point, probably 21," Armstrong replied when asked how old he was when he first doped as part of an ESPN documentary.

The exchange with US journalist Marina Zenovich appeared in a 90-second trailer released on Monday for a two-part documentary called "Lance" which will be broadcast by ESPN on May 24 and 31.

During the clip, in which former US Postal Service team-mates Tyler Hamilton and George Hincapie respond to the same question about performance-enhancing drugs, Armstrong, now 48, explains there are "a bunch of ways to define doping".

"The easiest way to define it is breaking the rules. Were we getting injections of vitamins and other things like that at an earlier age? Yes, but they weren't illegal. I always asked (what I was being given). I always knew, and I always made the decision on my own," he said.

"Nobody said, 'Don't ask, this is what you're getting.' I never, ever would have gone for that. I educated myself on what was being given, and I chose to do it."