How to disfranchise 10,000 citizens

Based on a visit to the Stats NZ site and a rough calculation, there are about 10,000 New Zealanders who will turn 18 between 20 September 2014 and 29 November 2014 (the latest reasonable date on which the government could have set the next election).
Admittedly, 18-year-olds have a relatively low rate of electoral registration and voter turn-out. But nonetheless, the PM's decision to go to the electorate two months early has denied the right to vote at this election to thousands of young New Zealanders.
OK, I know that there were well-rehearsed reasons for the early election, and that those young people who just miss out will get their turn in 2017.
I make this point, though, because no-one seems to think about the young when such decisions get made. If many 17-year-olds feel aggrieved about this decision, then John Key doesn't have to care about them – because they can't vote!
But, should we allow Prime Ministers to decide the date of the election? The arbitrary disfranchisement of young persons is another reason for fixed election dates.