The observation of actions performed by another person activates parts of the brain as if the observer were performing that action, referred to as the ‘mirror system’. Very little is currently known about the developmental trajectory of the mirror system and related social cognitive processes. This experimental study sought to explore the modulation of the sensorimotor mu rhythm during action observation using EEG measures, and how these may relate to social cognitive abilities across the lifespan, from late childhood through to old age. Three-hundred and one participants aged 10- to 86-years-old completed an action observation EEG task and three additional explicit measures of social cognition. As predicted, findings show enhanced sensorimotor alpha and beta desynchronization during hand action observation as compared to static hand observation. Overall, our findings indicate that the reactivity of the sensorimotor mu rhythm to the observation of others’ actions increases throughout the lifespan, independently from social cognitive processes.