…maybe I understand that not correctly… Please correct me if necessary!
Another big difference is that you can attempt to create a chain of a given length (e.g. 6 blocks in a row) with a number of coins (and aggregated coin age!) and you can push the chance for a successful attempt quite high by owning a high number of coins.
But in difference to security by PoW where you literally control the network (because you control the PoW process) by having > 50% of the hashing power (and you can sustain that attack, if you keep > 50% of the hashing power), you can only try to write some blocks in a row when security by PoS is in place. If you have spent the coin age, you have shot your wad… You can’t sustain an attack that is based on coin age if you consume coin age each time you create a block.
…and in addition: that attack would be costly, because you need Peercoins with coin age to pull off that attack. If you attack is successful, the value of your Peercoins is affected. It might be hard to make money with this kind of attack (just imagine someone might want to wait for more than 6 confirmations in case a large transaction is incoming!).
Technically it is possible - albeit not wise (economically speaking).
But as always it is not about absolute security - rather about relative security. And Peercoin’s security is relatively high…