Common Core's first major blunder is its treatment of nothing.
From CCSSI K.CC.3: ``Write numbers from 0 to 20. Represent a number of objects with a
written numeral 0-20 (with 0 representing a count of no objects).''
Long before children read and write, they speak, and long before they write numbers, they count and conceptualize. As young children, we don't count starting at zero, we start at one.
At what age can a child abstract that when you take away everything, you have nothing and that is represented by the number zero? Obviously, it's a concept you introduce in stages: everyone knows ``none'' or ``nothing'' before you abstract to the number 0. We don't claim to know what the appropriate age is, but we KNOW this doesn't belong in kindergarten, before counting is mastered. We'd guess that by the end of first grade, after learning about ``taking away'' and subtraction, every child should understand both the concept and perhaps the number. But the steps have to be clearly set out as part of the curriculum; otherwise, ``0'' will likely be glossed over by teachers in the classroom as something trivial and obvious, which it is not.
CCSSI's ``Mathematical practice No. 2'' is: ``Reason abstractly and quantitatively.'' The concept of none is the first major abstract idea in mathematics, and it needs careful treatment.
[CCSSI does in fact mention the sequence from concept of none to digit 0...in the Mathematical Standards for High School, p. 58. Which elementary teachers are reading the high school standards?]