She was three?

Well, that question was quite easy, then, wasn't it? "A Sphinx's brain matures faster than a human's then. Because she's speaking in complete, coherent sentences and everything." So, they were smarter than humans, for one. Which made sense, considering they were into riddles and things. "And maybe they mature faster, appearance wise, as well. Then they stay at a constant mid-age for a while. When they're at their physical peak." That would be favorable when they did get defensive guarding treasure. "Before their mental ability and physical fitness decrease again, with age." Old people, obviously couldn't lift as much as a thirty year old could. And they seem to be more forgetful when they're older.

But that was just comparing them to a human, of course. They could easily compare themselves to humans and have everything backwards. Or they could be compared to lions and have things differently. Or parrots... Or Knarls... Or Kelpies...