I have a hinny that I bought at 9 months, currently about 17 months old. Sharp as a tack. Really, just a mule with reverse parentage, with all the brains and baggage that go with being a mule. Can be a bit smaller than a mule, due to most jenny's being smaller than most mares. Ears likely won't be as long as a mule's, but longer than those of a horse.
My hinny is an absolute joy, and sometimes an absolute terror. As smart as working dogs I've owned. Intelligence near that of a pig, I'd say. And they won't ever let you forget it. And their memories are long--do something stupid or treat them badly and they will never, ever forget it.