When do Legendary Dragons start showing up? I am level 56, and I have read places where people are in the 40's and finding them, and some people say you have to be 70+. Does anyone have a definite answer?

50 for Ancient Dragons, so Legendary Dragons should be around that level.

I've heard it's level 78.

Yep, just checked.

bummer, i have to grind for hours to get achievement

They appear at very high levels because at lower ones they would be drammatically overpowered.

I don't mean to be contrary...but to be contrary =P
I just hit 40 and I've encountered Ancient dragons a hand full of times already. They're nasty creatures.
*shrug*

It is possible my two high level characters also had Ancient dragons before level 50, but my highest is 76 and there has been no sign of any Legendary Dragons yet.