Who Can Donate Blood?

You need to be healthy, a minimum weight (usually 7.5 stone, or 105 lbs).  Over the age of 18 and under the age of about 60 (if it's your first session).  If you have donated in the past without problems and are now over age 60 you may be able to continue donating, depending on your general state of health.

You musn't have any contagious diseases, or even minor illnesses (like the common cold).  Although blood is screened for things like HIV and hepatitus C, it's a risk to everyone if you donate knowing you may have one of these illnesses.  There are other grounds for exclusion (like having received a blood transplant yourself before screening for CjD was effective), or having had childbirth, tattoing, body-piercing or acupuncture in the previous 12 months.

You also must not be suffering from iron-deficiency (anaemia).  Even if you can't donate now, you may be able to at a later time, so it may be worth asking again in the future sometimes.