The short answer
Aged rice is stored for months after harvest, often a year or more, before it is sold. As it dries, the grain firms up, so it elongates more, absorbs more water and cooks fluffier and less sticky. That is why purana chawal is prized for biryani.
Fresh vs aged
| New crop | Aged | |
|---|---|---|
| Moisture | Higher | Lower |
| Cooks | Softer, can be sticky | Fluffy, separate |
| Elongation | Less | More |
| Water needed | Less | More |
Cooking aged rice
Aged rice needs a little more water and sometimes a few more minutes. If you switch from new-crop rice, add a couple of extra spoonfuls of water at first.