How long to wait before telling your child they're adopted?

[BreakfastBurrito]

So my friend mentioned yesterday that her little sister recently told her that she had originally discovered she was adopted by reading my friend's diaries. I asked how old she was when that happened and she said 4th or 5th grade (she's now 18). To me that seemed like a traumatically old age to learn both that you are adopted and that your older brother and sister weren't. I imagine if I was the parent of an adopted kid I'd break it to them around 5 or 6. What do you guys think?