That's in 'human years' which as we saw from Kogane is kind of half to quarter yokai years. Like Kogane's parents are at least 200 years old but sure don't look it, and Kogane is way older in actual years than 36.
And, well, totally legal in Japan to show underage tatas (or anything else) in a manga. Japan just insists you censor the hoohahs and wingwangs. But I am glad Mr. Dragonfly stepped in, did not need to see more.