Enforcement
Enforcement of this ruling would have proven to be problematic, since states not lowering the voting age to the age of 18 for state elections would have had to provide special federal-election only ballots to citizens between 18 and 20 voting in federal elections. States would have to maintain two sets of voting registries, one for those between the ages of 18 through 20 and another for those 21 and older.
This question became moot with the ratification of the Twenty-sixth Amendment the next year, which lowered the voting age to 18 for all elections in all states.
Though Oregon v. Mitchell affirmed the federal government's power to set a minimum voting age for federal elections, no case has tested whether the federal government possesses the power to prevent states from lowering their voting ages to below 18 for non-federal elections since the federal government has not tried to prohibit states from doing so, nor have any states tried to.
Read more about this topic: Oregon V. Mitchell