User blog comment:B214/Question and Answers 4/@comment-24331706-20150319150433/@comment-25032153-20150320155453

the reason that Rias calls the newer members of her Peerage children is becaus ethey dont have her knowledge of the supernatural world and of Rating Games which is what Devils learns early. Also they learn the magic to make them look like their youthful self like Rais mother is after they reach their majority which they have not said in the Manga though by the sound of it is when they reach over 100 years because Sirzechs is classified as a adult though he is over 100 years older then Rias but he also looks about the same age as their father in the Anime during school over seeing. they do saythat but some devils do have higher birth rates like the Phinex clan which has four children very close together.