I'm pretty sure I've read that the amount of sugar plays a part in crystallization as well.
Check your ice cream maker's instructions. You may need to replenish the ice at some point to achieve more solid ice cream.
...Which reminds me - need to send the kiddo out to pick blackberries tomorrow and have some fresh blackberry ice cream!
Crystallization is not a function of how old the ice cream gets. It has to do with how fast it's frozen. The faster it freezes the smaller the crystals. Slower freezing creates larger crystals.