I grew up bilingually. My father spoke Esperanto to me during the first 12 years of my life. The Wikipedia article (http://en.wikipedia.org/wiki/Native_Esperanto_speakers) gives some basic information. Unfortunately, other resources worth reading about that are in Esperanto. You might consider Jouko Lindstedt's "Native Esperanto as a Test Case for Natural Language" (PDF (http://www.ling.helsinki.fi/sky/julkaisut/SKY2006_1/1FK60.1.5.LINDSTEDT.pdf)).

I know that for many people, Esperanto is an "artificial" language. However, this distinction Esperanto and "natural" languages seems to me rather artificial (at least today).