Read it carefully. It does not bar a state from lowering the age below 18. It only bars the government from disallowing an 18 year old to vote. It would not stop a state from allowing an 18 month the vote.
If the state wants to lower the age, then the woding would need to change.
Figure reference would be made to reference 15 ear old and above not being able to be challenged based on age.
18 is the standard now based on the 18 year peg in there.