Experts recommend broaching such topics as young as possible. Recently such a case was discussed in a question/answer column in a Dutch national newspaper wher

Experts recommend broaching such topics as young as possible. Recently such a case was discussed in a question/answer column in a Dutch national newspaper where two experts answer such dilemmas. “When do you tell a child that their father isn't their biological parent?” was that week's question, and the answer was a unanimous “As soon as possible”. Don't hide it, don't make a big deal out of it; just explain it in terms a child can understand. Keep it hidden, and the moment they learn the truth (which inevitably won't be through their parents) you will have to deal with very strong feelings of betrayal and rejection that could have been avoided. It seems that the younger children are, the more readily they can just accept such facts. Keeping secrets will inevitably backfire and cause trust issues.