The surest sign of age is loneliness.

The line proposes loneliness as a more reliable marker of aging than physical decline. It suggests that what most distinguishes later life is not merely the body’s wear but the gradual thinning of one’s social world—through death, distance, changing roles, or the inward turn that can accompany long experience. Read this way, “age” is measured by accumulated separations: fewer peers who share one’s memories, fewer daily intimacies, and a growing sense of being out of step with the surrounding present. The aphorism also carries an implicit warning: if loneliness is the “surest sign,” then resisting isolation—through community, attention, and sustained relationships—may be a way of resisting one of aging’s hardest truths.