I would if I had an adopted child and as early as possible. Even if the child is too young to understand the meaning of "being adopted", I would introduce the concept so that he/she will take it as a matter of fact. I would think that it will help set the stage for answering future questions such as "where do I come from?". By being opened about the topic will also help encourage him/her to ask any questions or feedback to his/her parents if he/she is being teased in school. And very importantly, it makes it easier to assure him/her that he/she is being loved by the parents. Just my two cents worth of thoughts.
Studies by social workers have shown that it would be great if parents introduce the word "adoption" as early as possible so that it becomes a comfortable part of a child's vocabulary. Some believe that the best time to tell a child that he or she is adopted between the ages of 2 and 4. Personally, I'd do it around the same age as well. I'd start off my reading children's books about adoption to the child to make him or her comfortable with the idea. This book below is a great example:
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Yes, cos at some point they will/might know. So better sooner when they understand.
Yes, I would tell as soon s as they’re old enough to know