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.
The word abridged means it may not be changed or altered
Constitutional issues mainly, the constitution sets the age limit and the wording includes...”the states”. The Supreme court would have declare that the wording doesn’t preclude voters younger than 18.