I know where you are going. The next question will be: "Should it be legal for an 17.5 year old girl to enter into a sexual relationship with a 50 year old man?" And you will keep taking off half-years to see how far it goes.
Ask it again with 19, 17, 16... it's kinda astounding to me how people (a) base their concept of ethics on existing law and (b) are comfortable with the idea of hard cutoffs when it comes to consent.
I strongly support societal regulation; we should be very suspicious of the 50 year olds, and the 18 year olds should be warned. But there should be no legal restrictions.
Flip it: should it be legal for a 18 year old boy to date a 50 year old woman? If there's a difference in one's answer, endeavour to understand what that difference is, precisely