15 year old girl dating a 19 year old boy; if they have sex, is it rape?

The correct term is "Ephebophilia". A pedophile is attracted to prepubescent children. 15+ year olds are not prepubescent children.

No, for two reasons: #1, a 15-year-old has almost certainly reached puberty. I.e. If she has boobs it's not pedophilia.

#2, Even if she were prepubescent, to meet the official definition of pedophilia used by psychologists, he would have to be five years older, and he's just under that.