Final answer:
States have limited discretion regarding age qualifications for voting due to the 26th Amendment, which set a uniform voting age of 18 for federal and state elections.
Step-by-step explanation:
The States have limited discretion with regard to age qualifications for voting primarily because of the 26th Amendment to the U.S. Constitution. This amendment lowered the voting age from 21 to 18 in both federal and state elections. This constitutional change was influenced by the context of the Vietnam War, underlining the principle that those old enough to be drafted into military service should be allowed to vote. The amendment reads: "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 of age." This clause gives little room for the states to change the age requirement for voting.