There's a whole handful of families, mostly polyglots in Europe, who use Klingon as their home language because that's the one they have in common. And very shortly after that started happening, there were native speakers of Klingon as children arrived. This started decades ago, when the first Klingon books became widely available, so by now there are probably 2+ generation speakers.

The same thing has happened with Esperanto and Sindarin. Probably Esperanto has the largest number of native speakers because it's an auxiliary language.