Board Thread:Speculation House/@comment-14138255-20151213162620/@comment-26397825-20151215005314

Because they would be too young when Summer died to inherit Summer's Maiden powers, if Summer was a Maiden at all. If the minimum age is puberty, Yang and Ruby were too young so the powers would bounce to someone random. It doesn't just wait around for the girls to grow up, it just moves on.

It doesn't exclude them when they're older, so they would still have a chance of becoming the Maidens when they hit puberty, but at the time of Summer's death they would be too young and therefore not suitable candidates.

But that's only if there is a minimum age for Maidenhood.