I'm an adoptee, one of nine in my family (our family portrait looks like a UNICEF poster, no kidding

) and my son is adopted. We all grew up knowing it, and I explained it very simply to my son when he was about 3 or 4. No big whoop as far as he's concerned. He's 18 now, nearly 19.
The younger kids in my sibling pack hardly had to be told, all they had to do was show up at the breakfast table and look around
Anyway, one or two of us have gone on a bit of a "quest" in our late teens or early 20's, not out of any dissatisfaction, disillusionment, or anything of the sort. Just plain curiousity, really. The adoption records in my home state are sealed, so it was really a blind alley anyway.
I have to admit to a certain amount of "scientific curiosity" simply because I've got a few minor birth defects and have come up with a few illnesses and diseases that tend to be hereditary in nature. So, I'm a bit of a biological time bomb.
On the other hand, my 50 years on the planet so far have been one little surprise after another as far as that stuff goes, so, maybe I'll just wait for the next episode
In any event, telling them early, letting them know that they're kinda special because they got "picked out" (without making a large deal of this last point), seems to work well. Heck, I was about 11 before I figured out that babies DIDN'T happen at the front door in the arms of a social worker