Coins have to be left untouched for 30 days before they can mint. But what does the “90 days” refer to?
That should be the time it takes for a coin to reach its max age. Coin reaching 30 days start minting, then your chances to solve a block will increase until it reach its full potential at 90 days.
You can test the chance changes with this tool http://poscalculator.peercointalk.org/. Try setting here the age of transaction to 30, 90 and 200 days 
The “90 days” is very important to prevent someone from accumulating massive coin-age with even medium sized stakes. A patient attacker could wait years holding coins offline and then mint multiple successive blocks at once.
Way down the road of Peercoin adoption, the “90 days” could theoretically create a de facto minimum stake for minting because smaller stakes would never accumulate sufficient probability of beating the difficulty. This is one of the reasons why I’m interested in exploring protocol changes in which very large coin-age relative to the rest of network would actually begin to suffer decreased probability.