Objective Longitudinal and cross‐sectional studies suggest that affective instability is inversely related to greater age in borderline personality disorder (BPD). However, existing studies relied on retrospective self‐reports of perceived instability. We examined affective instability in everyday life in patients with BPD and healthy controls (HCs) by age in a cross‐sectional e‐diary study. Methods Two hundred and sixty female participants between 14 and 53 years of age (130 patients with BPD and 130 HCs) carried an e‐diary over 4 days. The e‐diaries emitted a prompting signal in approximately hourly intervals asking participants to rate their current affective state, that is valence (ranging from pleasant to unpleasant) and tense arousal (ranging from calm/relaxed to restless/under tension). Results Multilevel analyses revealed a significant interaction of age and group predicting affective instability (valence: F(1,255.6) = 7.59; P < 0.01; tense arousal: F(1,252) = 6.08; P < 0.01), suggesting that affective instability significantly declines with greater age in patients with BPD. Controlling for the number of comorbid disorders and BPD severity did not change the results, illustrating an inverse relationship between age and affective instability in BPD (significant interaction of age*group for valence: F(1,238.7) = 5.74; P < 0.02 and tense arousal: F(1,235.2) = 5.28; P < 0.02). Conclusion Affective instability during daily life declines with greater age in BPD. This decline is irrespective of comorbidity and BPD severity.