no hes actually 40 years old, he just regressed back into his 16 year old body while still mentally 40
His body is young but inside he's pretty much an adult. Actually though, the manga is mellowed out. In the novel, his experience was waaaaaaaaaaay worse. It's not the age that was fucked up, but it was the size. He arrived in that world basically in a very very young and small body. He would get fucked by these large beastmen, some even transforming in their beast form which are way larger and they would rape him mercilessly, like bloody. The worst part is that some had this kink of beating him as they rape him. Even if he was bleeding and injured, like broken bones and such, he would still be forced to serve costumer everyday, morning to night. He would be given barely any good food or sleeping area. The manga mostly cut these details, which I personally like, but in the novel, before he meet those two Platinum-quality Daddies of his, his life was a complete and utter hell. He absolutely deserve all the good things that will happen to him in the future chapters.
Beastmans live much longer than humans. Chika's 40 maybe be their 16
So this guy who was a sex slave is 16 and those beastmen are 35 years old? Wha-??
This is fucked up.