title_body: Is there anyone that has a conlang as their first/only language?
upvoted_answer: Yes. According to Wikipedia, Esperanto has 350 native speakers (data 1996). There is also a story about a linguist only speaking to his son in Klingon, but even though the child picked it up somewhat, later in life the child stopped speaking Klingon. Currently, he doesn't.
  
  Also, see Esperanto native speaker AMA on reddit and a blog I enjoyed about speaking Esperanto natively.