“The right of citizens of the United States, who are eighteen years of age or older, to vote shall not be denied or abridged by the United States or by any State on account”
The age is 18
The federal government and state governments are not allowed to deny someone the right to vote who is at least 18 on account of age. But I don’t see how that prevents a state from setting a lower age as the minimum if it so chooses.
That prohibits any law setting the voting age above 18. It does not prohibit any laws which allow those under the age of 18 to vote.