I believe that children should be raised with the knowledge that they were adopted. Other parents choose to wait for the right time to bring up the topic. Either way - it may be difficult for some families to find a way to feel comfortable enough to talk about a child's adoption story.
Here are several ideas on how to be more open within your family about the topic of adoption, Helping Your Child Talk About Adoption. There is also a bit of information on when adoptees may think about birth family or their adoption.
One tool that seems to help ease everyone into the discussion is a lifebook. Lifebooks are a great way to open up the lines of communication with your child. Here is a review I did on a great book that will help you understand the purpose of the lifebook and tell you how to create one for your child. Lifebook: Creating a Treasure for the Adopted Child
Suggested Reading on Lifebooks: