Fraction of OCs that are younger than log t = 8.5 as a function of galactocentric radius, shown only for clusters with masses between 250 and 2000 M . Orange squares show this fraction estimated from cluster counts in HR24 and corrected for incompleteness assuming a ‘warm’ kinematic heating model (Sect. 5.1). The blue square is estimated for clusters within 1 kpc of the Sun in HR24, given that this volume is expected to be ≈100% complete within this mass range (see HR24 Sect. 5.1). The grey line shows this fraction for HR24 clusters in our studied region (140° < l < 240°, |b| < 10°), uncorrected for selection effects.