The theory I ran with is that the midichlorines in a Force user are a double-edged sword. You use the Force, but the Force uses you back. Sometimes, it's benefitial, sometimes not.
Dark Siders, as a general notion, tend to obtain obscene amounts of power rather quickly. The midichlorines in that case, start acting as an autoimmune disease, attacking the body or ravaging it in order to sustain the power levels. As it's fueled by emotions like rage, hatred, and fear, and requires one to be on guard at all times, those stress hormones only add to the physical decay. They also recruit at a later age, possibly when the body is less able to handle the additional strain.
Lightsiders, since they grow their abilities much more slowly, give their bodies time to adjust to the added load. As they adjust, it benefits them in the form of slower aging and longer lifespan. Since the physical changes can be quite marked, it possibly presents another (though still ethically reprehensible) argument in favor of their tactic of recruiting in infancy. An infant or young child is better able to grow into the adjustments.