I think this may be appropriate for the thread here, in relations to Cutie Marks. Also, I have entertained the thought of earning part of a Cutie mark at a time, and I love the concept, though I don't think it makes very good concept for the already established characters. Hope you like Digibrony!
Well, you've never seen him any younger than a full grown pony, so you can't really believe that as fact, can you? Twilight is his MUCH younger sister, so we only see him at the age the Mane Six currently are, and the fully adult age his is now.