The United States population is still growing actually. We need to start dialing it back.
It doesn't happen overnight. There is a gap between the moment a country drops below 2.1 and its populartion starts shrinking. US fertility rate fell below 2.1 between 2010 and 2015. It will take at least 3 decades until population peaks and starts shrinking.
Japan fertility rate dropped below 2.1 between 1975-1980, and as a result population grew and peaked until 2008, and from now on it will only shrink.