User blog comment:B214/High School DxD EX/@comment-74.173.162.168-20160118130138/@comment-79.37.242.251-20160124004011

Yes they can change their appearance by will. For example Rizevim looks older than Azazel but he is younger than him and he does like this just because he likes appear in that way.

I think that simply they "choose" an age and than it stay like this permanently until they decide to change it again.