Where did you get that from? Does this mean that someone born in 1950 was born "in the 40's?" I would counter that the 20's starts when the leading digit is a 2, not when the trailing digit is a 1.The "20's" doesn't start until 2021.
Counting has always started at 0 for me. Just like daily time starts at 0000 ("zero hundred hours") and goes to 2359. But then, I'm an engineer/computer programmer.Counting goes 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 then 11; not 0, 1, … just saying...