Because your other questions today have had to do with the 26th Amendment, I assume that this question is related to that amendment as well. Therefore, I have added a date to your question to indicate what point in history is under discussion.
The major reason why the voting age was lowered at this particular time in history was the Vietnam War. The United States’ involvement in the Vietnam War was at its peak from 1965 through 1970. During this time, hundreds of thousands of American young men were drafted to fight in Vietnam. Many of these young men were between the ages of 18 and 21. These men who were being required to join the military, and whose lives were being placed in danger, did not even have the right to vote for the people who decided that there should be a war and a draft. This seemed unfair to many people. Because this seemed so unfair to so many people, the 26th Amendment was ratified in record time.
The primary reason to lower the voting age, then, was the feeling that anyone who was old enough to fight and die for their country was old enough to vote.