This is a bit misleading. By legal definition, according to Cornell Law School, "The legal age is also known as the age of legal majority. This is the age at which a person gains the legal status of an adult. The legal age is set by state law and can differ from state to state. However, almost all states set the base legal age as 18 years old." Thus, by legal definition and the definition of teenager (a child whose age has "teen" at the end of the age number), it is possible for a teenager to be an adult.
But there are few key observations I would like to make about this claim. First, all teenagers are not adults, since only people at or above the age of 18 qualify to be an adult in the United States. Hence, it can be argued that their is a distinction between being merely a teenager and a teenager who is an adult. Second, this claim does not ask in which country adulthood should be considered for. In different countries, there is a different age for adulthood. Third, this claim does not ask to which aspect if there is a distinction between a teenager and an adult. A number of factors such as physiological, and psychological, emotional can be differentiated between adults and teenager. Hence, there is no basis for the distinction outlined.
So if we go by legal definition, you would be correct a teenager can be an adult, so there would be no technical distinguishment. However, in other regards, I can not claim the same, and hence I think this claim is misleading.
Article Link:
https://www.law.cornell.edu/wex/legal_age