At what age is it appropriate to tell my dog he's adopted?

Be gentle. Let him know that he is extra special because he was "chosen."

just teach him to read and leave the adoption certificate lying around...

As long as you don't call him a son of a bitch..

*honk honk*

The funny thing is that pets adopt us.

I was adopted by an awesome kitty 3 years ago and my life is so much sweeter because of him.

When were you adopted by your dog ?

I don't know if it's the same for dogs, but I told my cat when she was about 6 months old.

doesn't matter. pack is pack at any age by any origin.