More than ten years. Now I know what you're thinking, 'Online roleplaying hasn't even been popular for that long!' or something the like.
Didn't at least a few of us play pretend when we were kids? Pretend we were knights battling demons or pirates plundering loot or spacemen exploring unmapped reaches of the galaxy. I did that a lot when I was young and I count it as 'Live Action Roleplaying'.