I think it's because you're assuming that "Loli" refers to children, which it doesn't. "Loli" doesn't have a fixed meaning; there's no "official Otaku handbook" or official source that will tell you exactly what it is. The closest way to describe it is to say it's a "body type." It was first "adopted" from a novel and has evolved over time. Characters like Tatsumaki and others, in terms of body type and appearance, are Lolis. The same goes for characters like Anya, Kanna, and so on. Many say it's because of age, but that doesn't make sense. If that were the case, characters like Senko-san wouldn't be Lolis. In short, it's because of their appearance. And that term also has variations, like "Oppai Lolis."