User blog comment:B214/High School DxD EX/@comment-114.79.12.100-20151203022921/@comment-27232228-20151203144748

Heavily implied, but as we know, DxD doesn't have much Taijutsu specialist other than the Great King (Koneko is one, but Sairaorg's stats and techniques are more refined). So it's natural for Airi to be his apprentice at an early age (dunno for the exact time).