Bad news for H.S. sweethearts where one turns 18 before the other.
Anyone who points out the important difference between psychos attracted to prepubescent children and leches attracted to post pubescent teens is excoriated. The psychos need to be offed. The leches given jail time.
When laws are actually debated and written out before being passed, exceptions are usually defined. In existing statutory rape charging in various states, the relative age of the two parties is taken into consideration, so that the scenario you described carries less culpability than the statutory rape of a 13-year-old by a 25 or 30-year old or above, which is often the case with "mommy's boyfriend."
That said, any penetration of a pre-pubescent child by an adult for purposes of the adult's sexual gratification, which biologically would be at least a 6-year age difference, should indeed carry the possiblity of the death penalty.