I have hard time reading "teenagers will be considered old enough to make decisions that will affect the entire country", how many 18 yo people are there any given elections? I don't say their votes doesn't count, but still in face of facts it is not real argument.

There is no way 18 yo is going to be president of any country. To be elected to parliament there is separate rule where you have to be 21 in my country (I assume it is quite the same in other countries). So even though you can vote when you are 18 you are not going to make real decision that can make impact.