Since members of Gen Z can also be known as "Zoomers" or "Centennials" (the same way members of Gen Y are known alternately as "Millennials" or "the iGen" --

Two terms that could be used to describe the youngest of Gen Y and the oldest of Gen Z could be the term "BiCentennials" or "Zillennials."

Additionally, since Gen AA ("Generation Alpha") immediately follows Gen Z, the youngest members of Gen Z and the oldest members of Gen AA could eventually become known as "AlphaCentennials." Of course, most of that subgeneration hasn't even been born yet...