Should they tell her that she’s adopted, or let her find out herself?

Should they tell her that she’s adopted, or let her find out herself?

“We have a 19 year old daughter who wants to do one of those DNA tests to learn more about our family. We never told her that she’s adopted because we’ve never had contact with her biological parents and she looks so similar to us. We thought it would do more harm than good by telling her.”