(First, a point of order:
Lady Sparks wrote:
the Lord/Baron/Vassal of the area
"One of these things is not like the others." A "vassal" is a sworn follower of a lord. He or she may be a nobleman, or even royalty, himself or herself, but "vassal" means "feudal subordinate," not "ruler." I suspect you meant "suzerain" or "sovereign," i.e. someone who has vassals.)
Anyway.
As with so many things, "it depends."
- How well is she liked (by the nobility and/or by the people)?
- What kind of education has she had (and do the nobility and the people know she's had)? (I don't mean classes, necessarily; remember the chapter in Prince Caspian about Caspian's upbringing.)
- Has she been present in, or better yet had a voice in, her father's councils or court? (If the answer is "regularly," I think she's more likely to be accepted than if it was "once, when she was presented five years ago"
) - Is there strong precedent for women as regents, castellans, or rulers in that area of your world?
- How competent is she, actually? (Elizabeth I got the throne of England largely because the other likely claimants were foreign and Catholic, but she kept it because she was a really strong and competent ruler, in addition to various other factors. Conversely, in the British Regency, if the Prince Regent had been a Princess Regent instead and had still been as corrupt a dissipated spendthrift as my reading suggests he was, I suspect Parliament would have deposed George III rather than let such a Princess rule as Regent.) (Or, to put it another way, the country's nobles and subjects would be more willing to get on the bandwagon of a successful regent who happens to be a woman than to support a failure whose failure is compounded by the female-descent weakness of her claim.)
- Is she betrothed or promised to any noble or foreign ruler? The nobility or the people might strongly object to someone who "will be" influenced by either one faction among the local nobility or, worse, a foreign power. (This would be less of an issue if the foreign ruler was a long-standing firm ally, or if the local noble was famous for honesty.)
To sum up: this can be made to work easily enough if you're willing to adjust your worldbuilding as needed. As a matter of history it'd be somewhat dubious (though, also, by the late 18th century a 21-year-old woman wouldn't be seen as "youngish," but rather quite old to be still unmarried ...). But this being a different world, you can change things that affect that.
Lady Camellia would be shocked at the suggestion. Though Sir Davion might be tempted to threaten such out of her hearing.