The Rambam writes (in his Introduction to Mishneh Torah):
ואחיה השילוני מיוצאי מצרים היה ולוי היה ושמע ממשה והיה קטן בימי משה והוא קיבל מדוד ובית דינו.
Achiah of Shiloh was one of those who experienced the exodus from Egypt. He was a Levite and heard [teachings] from Moses. He was, however, of low stature in Moses' age. Afterwards, he received the tradition from David and his court.
I can understand why Achiya wouldn't be considered a student of Moshe, if he was 1 year old when he left Egypt, he would be just a 39 year old when Moshe passed away.
But by Dovid Hamelech's time he was several hundred years old! Did he have to be that old before he matured into a full fledged scholar? Or was he simply not learning the whole time (kind of like Rabbi Yochanan Ben Zakkai who was a businessman for 40 years, learned for 40 years, and was on the Sanhedrin for 40 years)?