Left panel: age-activity relation derived for solar twins. The solid line is the best fit. The Sun is plotted with its usual symbol. Stars younger than 1 Gyr are represented as a single cluster (blue error bar) with mean activity and age = 0.6 ± 0.2 Gyr. The shaded region is the 2σ activity variability prediction band. Right panel: statistical significance of the AC relation as a function of the lower age limit. Black and red symbols are the results for the entire calibration sample and after a single round of 2σ clipping removal, respectively. The false-alarm probabilities reach ≈1% around 7 Gyr.