15 year old brains are not fully developed. No 15 year old, however brilliant, has the capacity to consent. Though I agree 18 is an arbitrary cut off and it should be higher.

Edit: Below I am replying to the reply to this comment here because apparently 5 posts in an hour and a half is enough to trigger rate limiting for me.

Yes, that is too young to make that decision. You can throw any hypothetical, emotional situation at this and the answer will be the same.