Because 5 year olds can't pay tax, enlist in the military, have sex, get married or do any of the other things that legally define them as an adult part of society. So it makes no sense to ask them to have a say in how that society is governed.
However, I'm sure you know this and are just trolling. Otherwise it's quite depressing that anyone over 40 could be quite so stupidly disingenuous in a serious debate.