When we had the 80's and the 90's, they were easy decades to reference. This new decade is a little harder. So, being the teen years, it's the twentyteens.
someone who is either 20 or in their early twenties but might as well be considered a teenager due to their immaturity and/or lack of life skills/experiences.
Heather: Joe asked me out!
Michelle: that guy from best buy? how old is he?
Heather: 20...only 7 yrs younger..
Michelle: eww he is twentyteen! you might as well add a teen to that age he is so young..unless he is sexy dont bother!