In this day and time it would be very difficult to avoid telling the child about their adoption. When to tell them would be the only real question and that would depend on variables related to the child's personality and circumstances.
It would be wise to explain what adoption is in a simple and practical manner at an early age. There are children's books to help do that and they could be introduced to a child along with other books about different topics. Beginning this way could help make understanding their own adoption a very natural process (unless parents are hyper-anxious and impatient).
Reading blogs on how to proceed could be very helpful, especially those that relate how the topic was presented to the adoptees in a loving, methodical way.