I am not sure about how long they should last but I would think 6 years would be well below life span. One very important step when changing coolant is to be sure ALL air is out of the system. This isnt helped by how difficult our cars are to burp. Any air flowing around in the system will cause erosion in the corners of the passages where the coolant changes direction. . I am not sure where yours started leaking but this might be something to check.
Blaine
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