Ah, I was waiting for this argument. There is certainly a gray area as to what point someone has the mental, social, and emotional capabilities to completely understand consent. I'm not a sociologist or psychologist so I can't give an informed answer. Our society seems to fix that age at 18. Are some 18 year old adults less mature than others? Sure.
That being said, as a society governed by the rule of law the benchmark has to be set somewhere. The gray area that exists is why we have courts who determine whether someone has the mental capacity to, say, stand trial, as one example.
I'm not sure how the "standards in this thread" would condone a 15 year old child consenting to insidious activity. You aren't being consistent.