You're right.
Long minimum staking age makes it more secure since in case you want to do a blockchain rewrite attack, you need to have valid private keys with 2 week+ coin age, those are a lot more difficult to come by (as most these attackers just buy their coins and dump it as soon as the coins reach the minimum staking age). It's also more secure since if you miss your chance of rewriting the blockchain, you have to wait again for 2 weeks to retry, rather than a few hours. This gives much better protection.
There's just one scenario:
If an attacked had managed to have such a private key and if he managed to do such an attack successfully, there would be less active staking coins to complete, at least in theory, so the attacker will need less coins to manage such an attack.
But in the end, as the security of PoW coins will depend on high hashrates and distribution of them, the security of PoS coins depends on a high amount of coins staking, and their distribution. I've tried doing some experiments on AC, not trying to rewrite the blockchain but trying to see if I could if I wanted to, the most consecutive blocks that I could manage to find has been around ~10. That's better than most other PoS coins but it's nowhere near PoW coins with well-distributed hashrates.
Anyway it's an interesting discussion there, thanks for the link. I think I'll join their discussion.