Local elections? Okay, that’s up to them. Federal elections? Not without a Constitutional Amendment, they’re not.
I stand corrected. The 26th Amendment prohibits discrimination in voting rights against anyone 18 or older, but does not prohibit voting by those younger than 18. In this
discussion of the 26th amendment, it was stated that although the 14th Amendment prohibited states from discriminating against males 21 and older, it didn't prohibit states from lowering the voting age below 21.