My random thought was:
It looks like the Ramia bloodlines is consistently producing female heroes. Doesn't that mean Kusa is also a girl, if he/she appears to be the "next" hero? At the very least the author follows the female line of the family tree.
Hmmm... on the other hand, we only know that Gertrude was a hero, so maybe that thought is bull. Unless we later learn that Kaiya is also considered a "hero" like Getrude was.
Leaving that aside, even if Ramia's descendants have had children in their 20s/30s, it is still possible for Ramia to have been older than that when she got her daughter.