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.
"...Have you eaten anything? Since, uh, last week? Or more relevantly last night, have you eaten the sorts of things humans eat or has that been overlooked in the confusion."
"A little, but we have more variety than fish all the time followed by weekly maidens."
"I ate things other than fish," he says. "Frogs. Sometimes deer. Sometimes birds, but they're very hard to catch. Other things. What do humans usually eat, then?"
"Sometimes any of those things, and also various plants, and some more kinds of meat. And milk and things made of milk, and eggs."
"Probably. Let's get you to the kitchen and you can try some things. And practice walking."
He tries getting up again. It doesn't work for long, again.
"...What if it takes me more than a day to learn how to walk as far as the kitchen?"
"Then someone will have to bring you things, but you'll probably get less variety that way."
Attempt to stand. He has that part more or less worked out by now, but the next step - so to speak - is still... yes, now he has fallen on the floor. He snorts.
"I sometimes find wearing shoes helps," mentions Carrabella. "Not with overwhelming consistency, but it can."
He looks around, locates the shoes, and manages a fairly effective slither-crawl in their direction, using his hands more than his legs.
It's mostly a hands task, so he has it pretty well covered; it's reasonably easy to guess things like which shoe goes on which foot and in what orientation.
The shoes are not a clear improvement, but Erian seems pretty content to keep standing up and then falling over until he figures out how the next part actually goes.
"I'll go ahead and ask the kitchen to prep a few things for you to try, since you don't know what human food you like."
And then having done that comes back to see how he's getting on.
He has reached a stage where leaning on a wall actually sort of helps, and is stumbling back and forth along the wall of their bedroom.
"What sort of food requires progress, anyway? Did they have to go out and catch it? I guess that can't be helped, I certainly can't hunt for myself in this state..."