My earliest u/s with my boys was 16w3d and it was OBVIOUS he was a boy. I think at 15w, there is soon room for error. However, if the baby is in perfect positin & very cooperative and you have a good experienced tech, I think you can find out definitively at 15w.
I work in the newborn nursery and I have encountered several couples who were expecting one sex and SURPRISE got another, it's gone both ways.