When should a child be told the truth about their conception?

Tweet

I've just learned that my niece is actually my step-granddaughter. Here's the basic scenario: Husband and wife have an on-again, off-again marriage, and while it's in the off mode, wife has a fling with husband's step-nephew (my stepson) and becomes pregnant by him. The husband says he will accept and raise the child as his own. Fast-forward to now, 12 years later, and biological dad thinks the child "deserves" to know the truth. Although it's not my decision to make, I disagree, and think the honorable thing is for stepson to keep his mouth shut. Kids this age have enough drama in their lives without this kind of news, and I think that if she is told they should wait until she is a bit older. My opinion is that most of the time when someone harbors a guilty secret, their motives for spilling the beans are selfish ones, to relieve their own burden of guilt, without considering how it will affect others. Sometimes ignorance is bliss. So, I'd like to know the opinions of others who may or may not have been in a simlar situation. If you found out at some point that the man who raised you was not your biological father, how did it affect you when you found out? How old were you at the time, and would have preferred not to know the truth?