After being printed first in Legends, the creature type was not supported for a long time.  . The subtype was retained because of the iconic dragons from Legends and was finally reused in Dragons of Tarkir.
The Elder Dragons of Dominia
The vast power of the ancient Elder Dragons of Dominia rivaled that of planeswalkers. They particpated in the interplanar Elder Dragon Wars, a conflict that shook the planes and killed most of their kind. Their title of Dominia refers to the multiverse, not to the plane Dominaria.
The Elder Dragons of Tarkir
The creature type returned after 21 years in Dragons of Tarkir to emphasize the importance of these dragonlords. While not members of the Elder Dragon race, the creature type was used to represent their great age and the respect they command (tying in with the theme of time travel of the block) and to pay direct homage to Legends's cycle of dragons.
- Ugin, being older than the Tarkir elders, can be considered an elder dragon in the same vein as the dragonlords of Tarkir (that is to say a "very old" dragon, but not a member of the Elder Dragon race). 
- Although many cards have "elder" in their name (for example Argothian Elder, Elder Land Wurm, Leaf-Crowned Elder  or Revered Elder) none of them have the subtype Elder.
- Although Niv-Mizzet is over 10,000 years old, he is not considered an Elder Dragon.
- Doug Beyer. (2014.) "Elder as a creature type", A Voice for Vorthos, Tumblr.
- Doug Beyer. (2014.) "Ideas of using the Elder type more", A Voice for Vorthos, Tumblr.
- Elder Dragons (comic)
- Doug Beyer. (March 2, 2015.) "WHY is Silumgar an Elder Dragon?", Voice for Vorthos, Tumblr.
- Doug Beyer. (February, 2015.) "Is Ugin an Elder Dragon like Nicol Bolas?", Voice for Vorthos, Tumblr.
- Doug Beyer. (December 19, 2007.) “Folk of the Non-Pines”, magicthegathering.com, Wizards of the Coast.