Question

Why did congress created the law that allowed 18 year olds to vote?

Answer 1

During debate of the 1970 extension of the Voting Rights Act, Senator Ted Kennedy argued that the Equal Protection Clause of the Fourteenth Amendment allowed Congress to pass national legislation lowering the voting age. ... However, the Court upheld the provision establishing the voting age as 18 in federal elections.

Answer 2

The 26th Amendment, the arguement was that if men were old enough to be drafted into the military they should have the right to vote.

here's a helpful link

https://www.history.com/topics/united-states-constitution/the-26th-amendment