One of my good friends raised a baby with a woman (baby wasn't his) and they went on to have 3 more kids. They didn't tell the little boy until he was 10. And as far as most people are concerned, this man is the boys bio father (I only know because he told me under the influence of alcohol on a night out last year). When they finally told him he had a better understanding and they told him nobody else knew his dad wasn't his bio father, but he could tell who he wanted. He told nobody. He said he wanted things to stay the same and he ignores the fact he has 2 dads. His biological dad doesn't exist to him, because he knows he's a jerk and doesn't want to be involved with him. So he has no false hope of his dad ever coming looking for him.
Personally, I think 10 is far too old. But these people kept it from everybody, partially due to the fact that the boys bio dad was abusive. Tell her when you think is the right time. If it were me, i'd make it known (to her) from a very early that she has a different dad. Tell her that her dad wasn't ready to be a daddy. If you leave it too late she might feel betrayed. Or in a few years time she might question why she looks different, or has different eyes or hair. She might ask if she's adopted or something silly. It'd be better to tell her than her find out for herself. Otherwise she might think you are keepingother things from her Who's surname does she have?
Anyone can be a father, but it takes a real man to be a daddy