The Good (Canonical) ending shows him on his deathbed after the Little sisters graduated college and had kids on their own. If he did only have 4 years to live, it would make no sense.
The fact of the matter is: Up to 21 years old, growth is controlled by hormones. After that, the body just degrades till death. Assuming you use the hormone theory, it's possible to age someone to as late as 21 before they start aging normally due to degradation.
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