You need to be really careful with terminology when explaining that attack. It isn't "equivalent to a 3% increase in inflation" in any way except that it encourages spending. You'd do better to phrase your argument like,   "Governments use inflation to encourage spending. They could also encourage spending by charging tax on movements of old coins".

I think this is an interesting academic exercise. I don't believe it has any impact on the real world.

As noted by others, this wouldn't be implementable because if a government wanted to try this, there would absolutely be some kind of warning that it was coming. Users would then start bouncing transactions around to keep their apparent age low. It'd be impossible to keep such a plan secret, if only because governments and central banks are set up to be at least a little bit transparent. Meeting notes from the UK MPC for example, are publicly available:

   http://www.bankofengland.co.uk/publications/minutes/Pages/mpc/default.aspx

Anyway, if a government decided it monetary policy was more important than Bitcoin it would simply ban usage of Bitcoin and force everyone back to state issued currency. I don't believe there would be any reasonable argument for doing so, because I believe in a purely Bitcoin based economy there would be no business cycles and thus no need for monetary policy.

If you think about recessions in the recent past (the last 30-40 years or so), how many of them were caused by something fundamental like a natural disaster or disease? None of them. How many were caused by malfunctions of the financial system? All of them.

If you build a more stable financial system the need for centralized economic planning to try and stimulate spending would go away. There'd be no justification for it any more.