How do I address questions from my 7 years-old adopted child?
First, it is great that your child is asking questions about their adoption. It means they trust you and feel comfortable talking to you about their adoption. The main thing to remember in talking to your child about adoption is to always tell the truth. Your child has a right to know their story.
Sometimes adoptive parents are reluctant to answer a question because the truth is unpleasant. For example, if a birthparent made an adoption plan because of a mental health issue such as an addiction or depression you might tell your child that their birthparent made an adoption plan because they were not able to care for them. If they press to know why, just explain that sometimes their birthparent has problems with drugs or depression. Explain that this does not make them a bad person, just someone who has a problem that makes them unable to parent even though they really love their child. It is important to tell your child that the birthparents loved them otherwise they will assume that the reason their birthparents placed them for adoption is because their was something wrong with them.
If you have an open relationship with the birthparents (which is ideal) I would also talk to the birthparents about how to talk to your child about whatever the issue is. Please remember you are not protecting your child by hiding this information, but it is perfectly okay to frame it in a developmentally appropriate way. Be sure, however, that your child has all of the information that you have by the time they reach adolescence.
In addition, it is important that you bring up the subject of adoption proactively if your child has not mentioned it in a while. This signals to your child that you are comfortable with the topic and they should feel comfortable discussing it.
One of the hallmarks of a healthy family is open communication. Research also shows that adoptive children who grow up in homes where adoption is freely discussed have the best outcomes.