It is unlocked but I open it occassionally. Even if I bought the mintcoins from the stock exchange and transferred them to my wallet, I would get an interest in an interval like every 20 days right?
I am not 100% sure what you mean. The "age" of the coins resets every time you move them. So like if you bought 10 days ago on an exchange, then transferred to your own wallet today, the age of the coins is now 0 not 10 days old.
Also, it isn't exactly every 20 days. It cannot happen until they are at least 20 days old but there is no guarantee that it will happen immediately at the 20-day mark. Most of mine happen at 23-25 days or so as I said earlier.
I moved it from the exchange to my own wallet something like 2 weeks ago, thanks for the explanation and have a nice day.