Amendment 26

"Section 1.The right of citizens of the United States, who are 18 years of age or older, to vote, shall not be denied or abridged by the United States or any state on account of age.
Section 2.The Congress shall have the power to enforce this article by appropriate legislation."

In this amendment, required voting age was lowered from age 21 to age 18. The argument behind it was that there are young men fighting for the country at younger ages than 21 and are not allowed to vote, but they are serving in Vietnam. This was of great importance to families who had sent their sons to war, and knew they may not come back. Obviously, the amendment passed because they saw that it was unreasonable that someone who was serving our country could not vote for the way they wanted the country to be ran. 

Here is a video portraying the intensity of the war to convince voters to pass amendment 26. 

This is an image of a protest that happened during the Vietnam War, supporting the right to vote for people 18 and older. The majority of the people in this image look like they are in high school, not necessarily parents of sons in the war.