Board Thread:Questions and Answers/@comment-16544958-20131218132129/@comment-3480156-20140217001900

Actually, I noticed this on Ohtaka's tegaki blog -> http://tegaki.pipa.jp/525880/24337832.html

According to the notes made on that first illustration, Kouen is two years younger than Hakuren and five years younger than Hakuyuu, so there you have it~