Well, I would say maybe 12 or 14, when they can understand the importance of keeping the secret.
I've read some fics where L&C's children were still very young (8-10 years old), they were planning to wait but usually at this age or 12 at most they would find out by themselves. I think maybe at psicological level they'll find out when they are ready. Because even if they want to wait a little longer to tell them, you can't keep such a secret at your own house, as careful as you'll try to be at some point you'll slip...
Just my 2 cents...
Carolyn
