= Solution
Many <open clusters> are weakly bound and are dispersed by <two-body relaxation>, <stellar evaporation> and <tidal heating>, including encounters with <molecular clouds> in the <galactic disk>, so their present age histogram preferentially retains young survivors. More massive compact <globular clusters> can survive much longer, while the released low-mass <stars> remain as old field <stellar populations>: \b[disruption removes clusters from the catalogue without removing their stars from the disc].
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