If you are receiving donated milk for you child, it is nice to seek out milk that is most closely "aged" with your child's age. For example, if your child is 2 months old, you would ideally find milk that was pumped and stored at a time when that mother's child was 0-3 months of age. In the early days, colostrum and higher fat milk can sometimes be more difficult to find. Even if the age of the babies varies greatly, it is almost certain that more "mature" breastmilk is still far healthier for a young baby than formula would be. Your pediatrician will likely tell you that maturity of the milk is a fairly insignificant factor. If you pediatrician is not supportive of breastfeeding in general, shop around because there are many out there who are.
**This is directly from the MilkShare website, under Store and Ship, the Age Appropriate.