Over the 20th century, left handers have increased as a fraction of the population. Left handedness may be relatively fixed as a genetic matter but in the earlier decades of the 20th century children were strongly discouraged from exhibiting left-handedness. As a result, many “natural” lefties learned right-handed behavior and identified as right-handed adults. Over time, however, the cultural suppression of left-handedness declined and the proportion of adults exhibiting left-handedness increased ...
Now suppose you take a random sample of people who died in 1990. In this sample, some people will have died old and some young. Among those those who died old, however, fewer people will be identified as left-handed because the old grew up in a time when left-handedness was suppressed. As a result, the old deaths in your sample will tend to be have more right-handed people and the young deaths will tend to have more left-handed people causing you to incorrectly conclude that left-handed people die younger.