Yes, anyone who is female (or wraps up in sufficiently mysterious shawls) and old enough could pretend to be a mysterious old lady, but the actual category of the creatures is a different matter, and one can often tell (though it's better to be on the safe side, when identifying them).
Mysterious old ladies can, in a certain sense, do magic - or help you do magic. Sure, if you decide to take the first milking of your new cow and leave it out in a bowl in the backyard with a garland of roses floating on the surface and then drink it all first thing in the morning without stopping to put on your shoes or comb your hair, nothing will happen. But if you do it because a mysterious old lady told you to, and it was a real mysterious old lady, and you followed all of her instructions? Then you will find that you will get whatever result she advertised.
But if you step into your slippers first or tug a knot out of your ponytail or drink the milk early or late or use daisies for the garland, you will get something else.
It's very important to be exact and careful about these things.
Compared to what happens if you disobey a real mysterious old lady, being occasionally made a fool of by some pretender to the title because your eyesight is going or you were too sleepy to note her lack of a proper mysterious aura is nothing.
"It is. But you're already working on it. Maybe spend the meeting sitting down if you're going to talk to foreign dignitaries. ...There are probably some foreign monarchs who want apologies about their daughters."
"It doesn't. Not directly. What it does is make it clear that you are concerned about how they're feeling and are in a position to not do it again."
"The families of the girls might want some kind of recompense. I can't think of a great argument for not giving it to them unless the treasury is in really bad shape, but if it is you won't be able to give everybody a non-insulting amount."
"Well - probably. He's had a human prince's upbringing. It might even be more politically palatable - you're older, but you're twins, so it's not a significant age difference and people have had more opportunity to get used to the idea that he was the prince because you were off lindworming. But if he prefers to be king he hasn't mentioned it."
"He seems to always know what things need doing and how to get them done," says Erian. "I wonder if I should ask him if he wants to be king instead."
She shrugs. "It's even a good idea. But if you do that we will still be married and I will not be queen and I kind of liked the idea of being queen."
"Monarchs in the course of doing their jobs get to exert a lot of power and throw around a lot of money. Using it intelligently matters more than doing almost any commoner's job intelligently. And I think I could do it intelligently. I have no reason to doubt Taphinieu's qualifications, though."
"He might not want me to. Also, if he's going to be king, he will have to get married so that he can have an heir, and then there will be a queen I know nothing about involved who might have her own opinions."