I'm pretty sure the trigger is just the first quest dragon fight, just after you hand in the dragonstone.
I'm deliberately holding off activating the dragons on two of my characters. They are amazing and quite a lot of fun to fight, but it's also quite annoying if you're trying to do something and get jumped by one. The first time I encountered a random one I had no ranged attacks ready (only short-range spells and melee weapons) so I was pretty much completely unable to fight it. I took shelter in a nearby cabin but there was some mage guy in there who attacked me, and then followed me outside where the dragon was still waiting. :-((