I just checked medical.net for reference. Quoting:
"Milk teeth emerge through a baby's gums during the first couple of years of life. By the time the infant is aged 2.5 to 3 years, all 20 of the milk teeth should have fully emerged. These teeth loosen and fall out between the ages of 6 to 10 years to give way to adult teeth."
And Leo-chan looks like she is around 8 years old.
So yeah, it adds up okay. These are anthropomorphic animals, but their bodies obviously perform just like the bodies of humans beings. You could say they are 100% humans, only with furry ears and tails. It's the usual Itou Hachi, no surprise.
Really don't understand why some people think there's a contradiction somewhere.