[Guvante]

Not a genius, but here is my take.

The ability to mint blocks without mining makes a long term attack harder (since even if you leveraged coin-age in your attack you have finite coin-age to work with).

This is because if you have 51% of the power you can beat the rest of the network when it comes to generating N blocks. However if someone mints a coin-age block you can't make that guarantee anymore. Note that this is especially painful because attempted takeovers like this require giving up valid blocks. (So if you tried to do a 6 block rollback you would need to give up 6 blocks in the case of failure)

So I would say that long term a complete shutdown of the protocol due to a 51% attack is unlikely, as coin-age would make it require more than just a lot of hashing power, especially as the protocol gets older.

However I think it may provide a weakness in some edge cases. I wonder what happens if someone tries to use a significant amount of coin-age to do a quick double spend (say spend 1,000 peercoin that are about 3 years old then use the peercoin age to create a parallel block chain to undo the spend). You would have to give up your coin-age in such an attack, but that could be a good trade depending on the rate that coin-age pays out at.

However to be fair, this would require a lot of faith in peercoin, since it appears the protocol calls for a significant timelapse to acquire coin-age. So the likelihood of it happening is lowered.