Can a 19 year old adopt a 16 year old?

No, not unless they were related in some way and the minors parents were dead. I have known of cases where older brothers/sisters have adopted younger children in the family when the parents died, but an unrelated 19 year old would have a hard time. They have to be able to show the ability to support the minor and reasons why they should adopt the 16 year old. A judge would decide and take the advice of the Department of Social Services for the state where they reside. A home study would be done and interviews to be sure the minor is not placed in a abusive situation.