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.
What is there a commotion about? If it were about her being alive she'd think they'd pay attention to her. Where are the princes?
It's quite a large room, and there is blood on more parts of the floor than there isn't. The blood is intermingled with fragments of lindworm skin. Prince Taphinieu is trying very hard to coax a maid with a mop into cleaning up the mess, but she doesn't want to go near it.
The maid bursts into tears.
"It's okay," says Taphinieu, "you don't have to clean it up, I'll find someone else." To Carrabella, he adds, "No one wants to go near the skin bits in case they start doing things again."
"I think they're not going to do anything else. But if I keep being the only one who thinks that, I'm going to end up having to clean this mess up myself," he says wryly. "Erian's in your room. I found him some clothes earlier and he put them on - that's when the skins started acting alive - but he stayed in the room because he's having trouble figuring out legs."
"It's much less of a mess. Still sort of untidy but not covered in blood and milk anymore."
"Walking's tricky. I'm barely qualified to do it, let alone teach it, unfortunately."
"Some people like it." Pause. "So we're married and your father is dead and I think you're generally acknowledged to be the elder brother."
"I think I don't want to be king of Mahlirou. I'd rather live in a swamp and eat fish and have a long lovely coily tail. But it seems like I don't get to do that."
"Well, people do sometimes eat fish. Otherwise... yes, that line of work is a bit inaccessible now."
"So if I've turned from a lindworm into the king of Mahlirou and I can't turn back then I guess I'd better learn how to be the king of Mahlirou. Apparently walking is involved."
"It's not strictly necessary. You could be king and carried around all the time in a chair. But yes, walking helps."
"I wouldn't like to make someone carry me around all the time in a chair if they'd rather be doing other things."
"You could pay them. Some people would probably rather carry you in a chair and receive money than not do it and not receive money. But you haven't been trying to walk for very long so you may yet manage it."