But
Nevill, it is fine for Aria if 14 is considered old enough to marry, no? Just kidding, Erd is fine to wait, he has several options available already:
There is Rin, who is older and whose assets are considered "S-rating" according to Erd himself, for not to mention she has an actual wingspan which can be manipulated to a limited degree and, lastly but not least, her oath can be used to eventually justify giving Mercant a heir (once her affection rating is maxed out, of course). Then there is Zayan, who is already willing to sleep with him for SCIENCE!
After that there's Mieren, who may be willing to assist us once she has been restored to life. Lastly, there's Lucy, who is probably a human princess cursed with a transformation spell.
You also can count Ellen from Ontoglia, who may probably be glad enough to spend some time within our HQ in exchange of restoring her sight.
In any case, none of this will matter until we solve Erd's latest ailment, but it's the thought what counts. For this reason I am surprised that C has not received more votes, knowing the Codex.
Nevertheless, Aria is not to be underestimated, if only because she's not desperate to prove herself and also because she knows how basic human anatomy works, unlike others.
Finally, by the time her aging is an issue, hopefully our Overlord will have more pressing issues, such as an Evil Empire to rule over, and one or two or even thirteen heirs competing against each other to gain his attention.