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?