The age of majority, which is eighteen (18) in the United states is considered the age at which a child becomes an adult, and confers the right to vote to that individual. This age was 21 for the longest period of time, until radical left leaning groups from the 1960's decided to push for a reduction in that age with the argument that if you were old enough to be drafted and die for your country you should be allowed to vote.
https://history.house.gov/Records-and-Research/Listing/c_016/
A constitutional amendment finally passed and was ratified on July 1, 1971
This was a huge mistake.
After this measure passed most states began lowering drinking ages and other regulations and laws to match this new standard. Over the last 50 years most of these same states rolled back most of these policies. Because of a huge public outcry over drinking and driving and health we have seen the rules for alcohol consumption gradually rise back to 21 across the United States. Further we have seen the sale of other items that relate to self-control like tobacco products also moving up to 21. There are valid reasons for these changes. A wildly dis-proportionate number of people under the age of 21 have demonstrated a total lack of judgement, and when it comes to self-control and acting in a manner that is responsible have totally failed to conduct themselves appropriately.
There is a transition when you become an 'adult'. Until this transition is complete, you are still subject to the rules and laws of the society you live in. That's right the adults get to tell you what to do within the confines of the laws created by society to protect you. As you continue to age most people get more conservative. As you take on more responsibility, get a job, acquire more possessions, maybe even have children you begin to realize that the idealism of your youth is not exactly a workable foundation for the society you live in. This realization finally struck all the 'hippies' living in the communes as they realized that everybody does not really want what's best for the world, and that many people are lazy and will not work if they don't have to.
So my question is why don't we roll back the voting age? Frankly if you are demonstrably too immature to drink alcohol you damn sure are too irresponsible to vote.