There shouldn't be an 'age of consent.' Trying to force human sexuality into a legislated box is always a bad idea and should be avoided at all costs. So long as everyone involved agrees at the time to engage in sexual activity and understands what they are agreeing to, they should be allowed to engage in said activity without the heavy hand of the law waiting to scoop them up and throw them in jail or place them on a barbaric 'sex offender' registry.
Gun to the head probably 15 as a bare minimum, though up to the age of 18 or thereabouts (however long they are living with their parents), the partners involved have to be of a similar age -I don't want some 40-45 year old man screwing a 15-year old girl.
If the 15 year old consents, why is it any of your business? Sounds like you need a hobby if it bothers you that much. I would remind you that the idea that 15 year olds are 'children' is a very, very recent idea, a result of society extending childhood beyond its biological scope into young adulthood.