I don't think they would be allowed to expand the vote to minors in federal elections. The constitution sets the voting age at 18.
Locally and even statewide, they can do what they want.
Actually, under the Constitution, if a State set the voting age for State elections to 16, that would automatically make the voting age for Congress be 16. Anyone eligible to vote for the lower house of the State legislature is eligible to vote for the U.S. House, and those voting for the upper State house are eligible to vote for the U.S. Senate. They could still require someone be 18 to vote for Elector.
