Your opinion that suggests immaturity of 18-years-olds as the basis of objection to lowering the voting age quite makes sense, but I think that the true reason for lowering the voting age is not the matter of maturity and immaturity but providing practical education close to real society.

 The reason for suggesting to lower the voting age to under the age of 18 is because the adolescent period is the period to study for going into society rather than they are mature. However, most of recent education is for passing college. Thus, by granting the right to vote, the chance of realistic consideration and choice can be offered to adolescents.

 Lastly, you said adolescent are not deliberate in voting and can't understand the pledges. However, This is the problem of not only adolescent but also all generations. Therefore, Rather than prohibiting the right to vote of adolescent, adolescent and adults should seek a healthy vote culture together.