Generation Z begins to vote in 2020... how will that change things?

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The electorate will continue to get more racially diverse and older as the last of the Silent Generation is replaced by the smaller and more diverse Z people.

This is probably the most important fact about them, and the only one we can really use to make guesses about their politics.

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They aren't as large as the Millennials and thus won't have as large of an influence.

We don't know what the birth rate will be like in the next few years, and we don't know when the media will declare that Gen Z is over (presumably in the next few years).

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To nitpick, Millennials ended in the mid 90's. The oldest of Gen Z voted in 2016. The book belonging to the OP is wrong, or at least out-of-step with many demographers and researchers.

2000-2001 is the most common cutoff.