Human social connections are complex ecosystems formed of structural, functional and quality components. Deficits in social connections are associated with adverse age-related health outcomes, but we know little about the ageing-related mechanistic processes underlying this. Using data from 7,047 adults aged 50+ in the English Longitudinal Study of Ageing, we explored associations between diverse aspects of social deficits and both perceived and physiological age acceleration, which provide complementary psycho-behavioural and biological mechanistic explanations. We created and validated a novel physiological ageing index using clinical indicators pertaining to the cardiovascular, respiratory, haematologic, metaboloic and cognitive systems using principal component analysis. Doubly-robust estimations using inverse-probability-weighted regression adjustment estimators showed that living alone, low social integration and high social isolation were risk factors for physiological age acceleration, with those who lived alone on average 1.9 years older than those who lived with others (95% CI 0.9-3.0 years older
32% greater age acceleration than people who live with others). However, social deficits were not related to accelerations in perceived age. Analyses were robust to multiple sensitivity analyses and maintained four years later. These findings provide important mechanistic insight that helps to explain the relationship between social deficits and age-related morbitidy and mortality outcomes.