I was born out of wedlock to a mom that was single by the time I was born, and have never met the biological father. She wrote a little picture book explaining what a biological father is, how it's different than my "Dad" (she had since married someone unrelated), and there was even a little photograph of him in there. And lots of stuff about how her and my dad love me a ton and how it's not a big deal.

So I grew up with the knowledge since basically the day I was sentient, and therefore it as much a part of my life as my fingers and toes. I almost never think about it. Last time I did was in the doc's office when she was asking about family history, and I had to remind myself that my dad's family history doesn't count.

So you have one vote over here from telling the kid from day 1, possibly in a format that they can come back to and explore / show to other kids when they ask (i.e. the picture book I got)