From a story standpoint, I am sure the only reason all other candidates are so old is, for her to decide that the roughly same age Prince is the best option after all.
As for the other points they have mainly been mentioned, Political, Social standing, maybe even thinking about marrying into a more wealthy family. There migth be a couple of Lolicons in there, but not necessarily and keep in mind while there were a few arranged marriages in the past, where the bride was underage. In Nobility that usually didn't mean there immediately was a wedding and the marriage consumed, they usually waited till the late teenage years at least.