At the beginning of the story, Cinderella is a maiden. She’s young, unmarried, and female; those are the basic requirements (some people might propose an additional rule, but I think it’s clear she meets that standard also). She’s also pretty, which doesn’t hurt. A very ugly girl might technically qualify as a maiden, but the villagers probably wouldn’t call her that. Villagers can be cruel.
Once Cinderella marries the prince, most people would no longer call her a maiden. Certainly not once there are little princes and princesses. You can be a maiden or a mother, but not both. And, since they live happily ever after, even if there are no children she will get older. At some point, she will be old enough that nobody will call her a maiden, unless they’re just trying to flatter her by implying she’s younger than she really is. Royalty get flattered a lot.
Had she never met the prince, and lived a lonely life, Cinderella would still eventually have been too old to be a maiden. At that point, the villagers would start calling her a spinster or “old maid.” This doesn’t occur at a set number of years; more like a state of mind. If the villagers still consider her marriage material, they call her a maiden. If they feel she’s missed the marriage boat and might as well start collecting cats, then she’s called a spinster. If she gets really old, they might start calling her a crone. She would have a lot of cats by then.
However, all this is old-fashioned terminology. The words maiden and (especially) spinster aren’t much used nowadays, since they reflect a mindset that women’s role in life is based on their relationship to a man. Today, most people recognize that women can do whatever they set their mind to, and whether they’re married and whether they’re still young, doesn’t have much to do with it.
If you like the idea of being a maiden, by all means go for it. But remember, maidenhood, like fame, is fleeting, so take advantage of it while you can.