Maester Aemon is not featured in A Knight of the Seven Kingdoms tales because the chronology of those specific novellas takes place decades before Aemon's birth or during his very early childhood when he was still an infant sent away to the Citadel to train as a maester. The wandering adventures of Ser Duncan the Tall and Egg happen years prior to Aemon taking his vows at Castle Black and crossing paths with his younger brother Egg again.