In the specific example that you gave, a 15-year-old (at least in any US jurisdiction that I’m aware of) does not have the legal capacity to “consent” (in the legal sense) to the kind of behavior that you’re discussing. In general – there may be rare exceptions, I suppose – a 15-year-old is a minor, and therefore determined incapable of such consent on his or her own. So an adult parent or guardian could be held liable, in whatever way is applicable, for negligence (for not even knowing about the child receiving this kind of treatment from others), for depraved indifference (for knowing, yet doing nothing to stop it) or for assault (for participating in this kind of treatment). Other charges may apply; I’m not an attorney or a cop.
However, adults of free will and apparently sound mind can elect to engage in – and even to legally solicit and pay for, sometimes – behaviors that an outside observer would call clearly abusive, even to the point of causing bruising and blood loss. (This is aside from the self-abuse that some practice in the name of religion, one primary example of which will be on display in a little over a month when Good Friday rolls around in the Philippines. For one example out of many.)
I may have some kinks, but this isn’t one of them. I know about things, though.
Once the abuse (if that’s what we’re going to call it) proceeds beyond agreed-upon boundaries, or certainly past a point where the victim can consciously and at least apparently willingly agree that it should continue, then it may be prima facie assault.