Within contemporary Western (post) modern societies, loneliness is seen as a problem that is particularly associated with old age. Much less attention has been given to examining variations in loneliness across age groups. We examine patterns of loneliness across adults aged 15 years and older in the United Kingdom using data from the European Social Survey. We first consider the prevalence of loneliness among the adult population; then the relationship between loneliness and a range of key risk factors, and finally the relationship with age for each of our risk factors. Loneliness demonstrates a nonlinear U-shaped distribution, with those aged under 25 years and those aged over 65 years demonstrating the highest levels of loneliness. Depression is associated with loneliness for all age groups. Poor physical health is associated with loneliness in young adult and midlife but not later life. For those in mid and later life, the quality of social engagement is protective against loneliness, while for young adults it is the quantity of social engagement. This indicates that different factors may endow vulnerability (or protect) against loneliness at different stages of life and suggests that preventative strategies or interventions that reflect these variations need to be developed.