According to the US Constitution. Voting is a right for all American citizens over the age of eighteen.
According to the Constitution.
Voting is a right for all American citizens over the age of eighteen.
OK
The first word group is a fragment because it lacks a subject and a verb. One possible revision is to attach the fragment to the complete sentence with a comma:
According to the Constitution, voting is a right for all American citizens over the age of eighteen.
For more help, see section 19.