When a 48 year old buys sex from a 15 year old then we can treat it exactly in the same manner as if a 48 year old would want buy e.g. inherited real estate from the same 15 year old - we simply acknowledge that the minor isn't able to grant informed consent and is likely to be exploited, so we don't consider the transaction consentual.

A 68 year old buying sex from a 18 year old - why not, it happens e.g. in marriage/"golddigging" scenarios? I mean, I can at the same time morally dislike some behavior and also acknowledge that I do not and should not have any right (moral or legal) to prohibit that behavior if they wish to do so.