People grow at different rates, duh. My own brother is two years younger than me, and he's both taller and heavier than me, and looks about five years older, too. Why would their looking different matter? None of that explains why Acnologia "knew" E.N.D. when he didn't even recognize when E.N.D. was crawling on his body earlier in the story.
Face it. The only people who knew E.N.D. were Zeref and Igneel, no one else. Acnologia's statement never implied he knew E.N.D., only that he wasn't afraid of a demon. Knowing E.N.D. is a demon is not proof, because fucking everyone knows that E.N.D. is a demon. They just don't know what he fucking looked like.
Face it. The only people who knew E.N.D. were Zeref and Igneel, no one else. Acnologia's statement never implied he knew E.N.D., only that he wasn't afraid of a demon. Knowing E.N.D. is a demon is not proof, because fucking everyone knows that E.N.D. is a demon. They just don't know what he fucking looked like.