When did you understand the concept of adoption?

In my open adoption, the kids are almost 5 and almost 2. They are told frequently that they grew in their birth mom's tummy and that mommy and daddy adopted them and how special and wonderful their stories are. The 5-year old seems to have a vague understanding, and of course the two-year-old is way too little to understand. We believe that it's best to totally normalize adoption from the get-go, especially since birth parents are around so often. My question for adoptees is when did you understand what adoption was? If you were raised in an open adoption, when did you understand that your birth parents were your birth parents?