Her age isn't really important, but she's still the "cute" female character that you already know, uses the colors traditionally associated with valentine's day, and the other 2 choices are grown men (Who frankly aren't even attractive) and who you don't know and don't have very desirable colors for a valentine's glamour, plus a lot of people have already admitted to just choosing Astrid because they wanted red clothes and couldn't even be bothered to read the site that says that the outfits are dyeable, let alone actually read the quest dialogue and make an informed decision.
A lot of people voted for a color and a "cute" girl, and not an idea.
