User blog comment:Morganaforever/Finale/@comment-213.233.103.2-20111225233003/@comment-5036924-20111228171804

It's just a logical assumption given that Aithusa was a newborn dragon Merlin brought to Kilgharrah, who is unlikely to abandon the only other dragon in existence. Aithusa would presumably be too young to leave Kilgharrah's care at least for some time, so Kilgharrah would have to have been taking care of him for at least a short period of time after Merlin gave him to him. They may not continue to stay together, or indeed may have already separated, but Aithusa would have had to have been in Kilgharrah's care briefly.