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.
When I was on the grand jury we had a case presented to us, at the request of the girls mother. It was a 16 year old girl and an 18 year old guy. They had dated for a year. The mom found out and called the cops.
Half the women on the grand jury said they had an 18 year old boyfriend when they were 16.
We did not indict. This case was where the mom was pissed off that the guy dumped her daughter.
We had plenty of 30 year olds with 14 year olds as well. Those were indicted without any debate.
Juries are pretty smart.