What do you think of this?

My dad adopted my oldest sister and paid for her to have our last name. My mom didn't want to tell her that my dad wasn't her real dad, but my dad wanted her to know. They told her around the time she was nine or ten, she didn't take it very well. I think younger the better to tell children and continue to tell him as they get older. I think it will be less of a shock, this is just from my family experience that is.