How do I explain to my little sister that she's too young to have ?

Don't think I'm being an annoying and protective big sister, but she's in first grade, and told our mom this morning that at recess her "boyfriend" asked her to come over his house so he can "take her down". The boy is also in first grade, and for some reason they were taught what that implies. How can I explain to my baby sister that she shouldn't do that until she's older?