you can run into them after 40.. I have slain quite a few of them.. but honestly, theyr not THAT much more powerful.. so its not even worth the excitement. other then the fact that they look great.
I met one at level 35 and it was quite a shock I can tell you.
I usually exit the Arch Mage's quarters by climbing the stairs to the top of the tower so I can fast travel. Going out the normal way means exiting to the Hall first (loading) and then exiting to outside (loading again). Going to the roof means just one loading event.
So I open the door only to come face to face with an Ancient Dragon. Didn't half make me jump!