Minors under 21 lack the mental capacity to vote. When the age was lowered to 18, We did not know about the brain as much as we do now. Minors are mentally incompetent and cannot make decisions for themselves until they are 21, And the brain isn't fully developed until 25. That is why the voting age should be raised to 21 or even 25.
The voting age was lowered from 21 to 18 in 1971 after young citizens were drafted and forced to serve in the army, Without having the right to vote for a President. However, There has not been a draft since 1973, Meaning nobody is forced to serve in the military anymore. Because of this, There is no valid reason why the voting age should be as low as it is now, Since studies have shown that 18 years olds do not have the mental capacity to make the same decisions as 21 year olds. You must be 21 to drink, You must be 21 to gamble in certain states, And you must be 21 to purchase a firearm. 18 year olds are still minors in many aspects of life.
There are plenty of 18-20 year olds who know more about politics and common sense than people over 30. The problem is that some people who have zero sense of what their voting for get a chance to vote. There should be an IQ test before every voting event. So there is a clear line which people can vote and who not.