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In epidemiology, this occurs often: Old people are less prone to the influence of risk factors. For example, the Framingham risk score, which tries to estimate cardiovascular risk, gives 8 or 9 points to smokers in their twenties and thirties, but only 1 point to those in their seventies (more points correspond to higher risk).

While several effects, including some selection effects, may contribute to this, the main reason is that the old have less to lose. Their baseline risk of most diseases and death is higher due to their age, and there is only so much "risk substrate" or "potential" or "residual disease-free life" left for any other risk factor to occupy.

Is there a word for this phenomenon?

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    $\begingroup$ "tough old bird" is idiomatic. Some sources claim that it implies females, but I've heard it applied to all genders. $\endgroup$ – Nick Cox Aug 1 '13 at 14:09
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Survival bias and competing risks. Also, elderly having a high value of a risk factor who have not been affected by that risk factor have demonstrated a robustness to that particular factor in general. This is why age $\times$ risk factor interactions can be important to pre-specify in a model.

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Age $\times$ risk factor interaction are, as mentioned, important. Some risk factors may have a different form of association in later life. Low values of some risk factors can be associated with mortality due to them being possible indicators of poor health in later life, leading to a u-shaped relationship. There is an interesting article by Abdelhafiz et al that discusses this phenomena:

The U-shaped Relationship of Traditional Cardiovascular Risk Factors and Adverse Outcomes in Later Life

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