The tendency of old age to the body, say the physiologists, is to form bone. It is as rare as it is pleasant to meet with an old man whose opinions are not ossified.

The remark plays on a physiological claim—aging bodies tend toward calcification—to craft a metaphor about mental rigidity. “Ossified” opinions are those that have hardened into fixed dogma, no longer responsive to evidence, experience, or changing circumstances. The speaker suggests that while physical stiffening may be natural, intellectual stiffening is a regrettable (if common) habit: many people grow older by defending settled views rather than revising them. The closing clause—“as rare as it is pleasant”—frames open-minded old age as both uncommon and admirable, implying that wisdom is not mere longevity but the continued capacity for curiosity, flexibility, and self-correction.