Wrong again: it seems that some states of the US have an age of consent of 18, although there is frequently a close age exemption clause which would apply to your boyfriend. Turkey apparently has an age of consent of 18. I've also made the assumption that you are in a heterosexual relationship: sometimes the age of consent for homosexual sex varies from that of heterosexual sex.
The answer is, then, that your boyfriend is probably not breaking any laws but that it depends on where you are.