Ok, the first thing to know is that new cells enter the blood stream twice a day every day. So, if you were to graph out the ages of the red cells in your blood stream, you should get a straight horizontal line . In principle, the same number of cells which die off today will be replaced by new cells today.

Granulocytes - usually liver between 7 -10 days Lymphocytes - varies by type. Some live 30-60 days; others for years perhaps for a lifetime. Monocytes - usually 5-7 days Platelets - usually 10 -12 days These numbers do not include the amount of time it takes for them to mature in the bone marrow. It only reflects the time they live in the blood stream.

These numbers assume that there is no stress on the particular cell. For example, granulocytes may only life 2 days if there is a significant infection. Platelets may only live a few hours if there is a bleeding episode.

Aspirin is only biologically active for about 20 minutes. So, those platelets which are in the "area" of the aspirin are permanently prevented from using a specific enzyme pathway. Not all platelets are close enough to the aspirin during its biological activity to be damaged so this does not happen to the entire population of platelets. It also doesn't happen only to young or old platelets - it happens to those which are in the area.