I still didn't know how the NRS mint works.
Do I need open wallet in order to mint?
I have some NRS with coin age > 20 days, and I open the wallet but nothing mint.
It could take a while. Just leave it open. I've had to leave my wallet open for a couple of days.
It starts at 7 days and i believe the max is 14, so in that interim you will eventually roll for a pos block. If you get the block the counter on the coins resets to 0 and the process starts all over again. I believe the timer starts from the time you move the coins into your wallet. I have found more through pos than i have mined, i have bought a few nrs though

Keep in mind that once a wallet address hits 14 days that doesn't mean it will POS immediately, that just means that is the maximum coin age. If I have an offline wallet sit for 6 months then online it, it will be on the same footing as one that has been sitting for 14 days.