People who donate blood regularly are screened for disease, so people who continue doing so have less disease by definition. And if they do have disease it's of

People who donate blood regularly are screened for disease, so people who continue doing so have less disease by definition. And if they do have disease it's often identified earlier. People doing some high risk activities are barred from donating blood completely, which again would have a positive effect on life-expectancy of those that donate. It would be really hard to tease out causality here.