Yes. Democracy is predicated on the idea that everyone's interests count. This creates a relentless pressure to extend the franchise to everyone even moderately capable of exercising it.
16 year olds have interests, and they are certainly able to express them. They should therefore be able to express them where it matters - at the ballot box.
(As for the idea of a "responsible vote", I regard this as simply leftover aristocratic bullshit. Government isn't there to make responsible choices; it's there to make our choices. Its responsibility depends on us, the votes - but we can certainly choose to be "irresponsible" (e.g. by not sucking up to the US and invading Iraq, or by taking a stand on climate change, or by taxing business to support a welfare state) if we so choose).
