The American Academy of Pediatrics (AAP) has issued a report stating that parents should tell their children if they are adopted, and that doctors should work with adoptive parents to help them raise their adopted children.

“Children who join families through adoption can have unique medical, educational, developmental or behavioral issues, and it is important for both pediatricians and families to be aware of the psychological challenges that many adopted children experience.” (Medpage Today)

The AAP contends that as adoptive parents and their children encounter these psychological challenges, that pediatricians can be a great resource.
The report also underscored the importance of using positive adoption language; for example, adoptive parents should not say their biological parents gave them up, but rather, they created “an adoption plan” for the child. Also, the AAP pointed out that by referring to biological parents as “natural” parents, the adoptive parents are implying that their family of adopted children is unnatural.
Though the AAP gave no timeline to indicate when parents should tell their children, they did state that kids do not generally recognize differences between families until age 3.
The authors of the AAP report conclude,

“it is becoming increasingly important for pediatricians to be aware of and knowledgeable about adoption…

[They] play an important role in helping families deal with the differences, the losses, and the many other issues surrounding the adoption of a child.”

Citation for the AAP report: Jones, V. F., & Schulte, E. E. (2012). The pediatrician’s role in supporting adoptive families. Pediatrics. Advance online publication. doi:10.1542/peds.2012-2261