That would be child-led weaning. Some kids really do wean themselves when they're ready, but it may be much later than the mother wants! I know a woman who allowed/allows her children to wean themselves and her daughter nursed until 5, her older son is now 5 and still nursing (along with his 2.5y/o brother, and the mom being pregnant again).
I think there can be gentle ways for the mother to encourage weaning, though. A friend of mine established a special nursing chair, so they HAD to go to the chair whenever the daughter wanted to nurse. If she was too consumed with whatever else she was doing, she'd sometimes decide it wasn't worth it to go all the way to the chair. Or the mom would say "quick noonies" (their word for nursing) and they could nurse for the count of 10, or something like that. Of course distraction/redirection too. I'm not trying to wean Solomon, but sometimes if it's not a good time to nurse I distract/redirect and he's usually fine (I do it BEFORE he actually asks though, when I can see the signs that he may ask soon).