Oh, there are two very old ones I can find for you. I don't know much about the history, so hopefully somebody else can explain them a bit better. But as far as I've ever read, these are among the oldest animated "features" you can find, though I don't think they are the oldest.

Both were created by the same person, Winsor McCay, from Western Michigan (my home state!!)

In 1914, he produced an animated short film called Gertie the Dinosaur. (animation starts around 7 minutes in.)

Then in 1918, he produced an impressive short film called The Sinking of the Lusitania.. The animation is stunning, even if the account of the incident is...not wholly accurate.