I actually told my daughter when she was 6 1/2. I have no idea how to tell a grown child, although I don't imagine either of yours will be shocked, kids are more attuned and understand much more than a lot of us give them credit for...

Why not print out a BP fact sheet, sit them down and say something like, "The name of my illness is Bipolar Disorder, I think you 2 are old enough to understand and accept this about me, do you have any questions?"

Don't go scaring them with genes that may not even be a problem, but answer their questions as directly and accurately as you can and encourage them to enquire at anytime.

Leaving the lines of communication open regarding info about bp will prove beneficial, should either of them become worried about being symptomatic.