Most children are unable to make informed decisions about voting, and use emotion rather than logic and facts...
If even one child existed who stood to contradict the claim endorsed by me, and if that child was denied the right to vote as a result of the law following as a consequence of such thinking, how would it be justice to that individual child?
Unfortunately for your point, 'female' is a permanent condition, whereas age definitely isn't. Unless you can stop Time, that is.
:^)