This would not stop teenagers from using their votes in the same way as a protest vote. Even people who are 16 and 17 will know the policy of their government and will be just as likely to vote on the basis of that policy regardless of whether they can influence it in national elections. Indeed teenagers tend to be rebellious against authority figures so it would seem much more likely that they would simply use their vote in protest, as a result they may well even be more likely to vote for parties that are extremist rather than simply going for the opposition to the government.