I don’t have a wife or kids, but I have heard this question discussed a number of time before, so take my thoughts for what they’re worth. From those discussions, there’s pros and cons to telling a child any age–and the pros and cons are often age-specific.
First off it’s worth taking a look at your motivation for wanting to tell your children. Is it something they needs to know now, or can it wait a bit?
One potential downside until waiting until children are 18, is that they sometimes are resentful that you kept a secret from them. So some parents do opt to tell their children around adolescence, since they can say they were waiting for the child to be old enough to understand–just like other “adult” things they may start discussing at that age. They can also rationalize that children are also mature enough at the age to be discrete, if that’s an issue.
But of course it depends on the individual child and your situation.
