We grow with years more fragile in body, but morally stouter, and can throw off the chill of a bad conscience almost at once.

Smith contrasts physical aging with a hardening of the moral sensibility. As the body becomes “more fragile,” the conscience can paradoxically become tougher—less easily chilled by guilt, quicker to rationalize or forget wrongdoing. The line reads as a wry, skeptical observation about self-justification: experience and habit may not refine virtue so much as build psychological defenses that let one “throw off” remorse. In Smith’s aphoristic manner, the sentence compresses a moral critique of maturity into a single ironic turn, suggesting that what we call moral strength may sometimes be mere callousness.