Is this an issue?

My adopted sister was twelve when my family choose to adopt her. After about four months, she understood everything, but spoke very broken English and could barely read and write. She and I spent two summers working really hard and now she's getting four A's and two B's in the grade she'd be in if she were in Russia. The older the child, the harder it is. It means more vocabulary, a pretty high reading level, and learning how to spell relatively long words. On the other hand...I have a friend, Masha, who came her when she was four, started kindergarten when she was five, and has been an excellent student all the way through school. It only took her 8 months to learn English.