But, the older constructed language, Esperanto (invented in 1887), actually has at least one second generation native speaker (who is the child of parents who were native speakers of Esperanto). As the report from a British Esperanto conference explains:
[A] genial 35-year-old man by the (actual) name of Rolf Fantom. . . . was famous for being that rarest of human incarnations: a second-generation native Esperanto speaker; that is to say, his mother’s parents met through Esperanto and this was their common language; they thus bequeathed Esperanto to their daughter who, in turn, brought up young Rolf to speak it as his denaska lingvo.It turns out that there are even third generation native speakers of Esperanto: "Nils Martin Klünder’s great-grandfather learned Esperanto, taught it natively to his kids, who taught it natively to his kids, who taught it natively to Nils."
There are an estimated two thousand native speakers of Esperanto, all of whom are multi-lingual, among them businessman and political activist George Soros.