Imagine your child is going to school or even become a friend with child who happens to have same-sex parents.
When and how would you explain it to your kid?
or... ?
There's not a definite age, you will have to figure out if your child is mature enough to process something like that. Normally they'll find out on their own, you shouldn't feel obligated to force them into having knowledge of something that is totally irrelevant for their age. Also, you don't want to say anything that will make your child not like their new friend anymore because they come from a different family. Just let your kid know that everyone is different. Not a single human on this planet is the same as someone else. This applies to everything; personality, race, religion, and orientation. Then just elaborate a little bit on all of those and hopefully your kid will understand it.
I know I'll get thumbs down for this but this is the truth.
Don't go into detail, but just say, these are people who do bad things. Tell her not to talk about it with other people because they don't understand. And just tell her those people do bad things.
Any age is okay because if it's an age too young for sex, they can still hear that this situation is ''bad'' so that they realize it when they grow up