Donated red blood cells vary in how old they are, that is, how long they have been stored since being collected from donors. Blood that has been donated is stored for a maximum of 42 days, after this time it is expired. That means that red blood cells that are given to patients as a blood transfusion can be anywhere from a few days old to 42 days old. The average age of blood that is given as a blood transfusion in this hospital is 21 days old.