So, my point is: the argument against sleeping with someone for being too young it is based on the idea of the older one taking advantage of the younger, right? Like, if you are too young you don't really know what you are doing and therefore a person with bad intentions can use you in a way you don't really want.
Said that, and understanding that every case is different, if we would discussing it from the legal point of view it makes sense to have some kind of barrier separating "too young" from "old enough". This is, we consider than, from a certain moment, a person should be mature enough to being able to fend by themselves (Of course an 18 year old person can be taken advantage of, as a 40 year old person can also). And if we are going to choose a more or less arbitrary age to decide when people is mature, it make sense to choose the same age for everything.
I don't know if relationships with a big age difference (I did not know the expression May/September hookups, thank you!) are something intrinsically different where the society, as a whole, as to take a stand and decide what is good and what is bad. In general, when it comes to relationships, it should be mostly up to the people involved in that particular one and not to anyone else to define its terms. The law (or the society) should be there to ensure that everyone is respected as an individual at every occasion, that's it.