Hard to say. I can say its not 14 or even 16. It also depends on the actors involved. Two sixteen year olds is different from a 16 year old and a 30 year old. I suspect you would like it to be a nice tidy guideline but it can't be. IMO biological adulthood is not the bar to be used. I'll bring up the "obscenity" example as I have in the past in these types of situations. I don't know if I can define it, but I know it when I see it.