http://www.brittanyshope.org/brittany.html
Read this story, but get some tissues ready beforehand.
I can tell you personally that we adopted a much older child when our youngest was 15 months old. She remembers no life without her big brother. Of all my children, her transition to having him in her life was the quickest. Literally, he was wrapped around her finger within 3 days of coming home. They have been inseperable since that point.
There would likely be a few more adjustment issues with a child closer to your dd's age. But, I think her adjustment would be swift and easy. His adjustment will be harder than hers.