I think that the main thing preventing teaching category theory (the basics that is) early on (and I do mean first year undergrad) is historical reasons rather than any intrinsic difficulty of the subject matter.
In fact, in many respects category theory is easier than group theory (which is first year material in some universities, but is more commonly second year). The reason being that you can draw many examples of small categories while you can't really draw small groups. You can write out their multiplication tables but that's not the same. Diagram chases can be done by anybody and provide a visual way to argue algebraically. In short, category theory offers visual ways present arguments and proofs.
There is a lot to gain from having the language of category theory at hand from the get go, though I'm not aware of universities that teach category theory as a first year course.