Their longevity is dependent upon several things. The major item that effects how old they will get before dieing is your medicine tech level. At level one typically they will live till the mid to upper 50's. Level two mid to upper 60's and level 3 mid to upper 70's, but I have had villagers live to 102, and have read one person had a villager live to be 103.

Of course your actual mileage may vary.
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The VI