Internet-era colloquialism.
奔 (bèn, 'to head toward / approach a target') + age decade. By extension:
奔四 (pushing 40),
奔五 (pushing 50). The standard reading is bèn (
奔 in the 'approaching an age' sense, per
现代汉语词典); bēn is widely heard colloquially. Carries a wry, slightly anxious tone about adulthood milestones.
