This study examined the causal relationship between knowledge, attitudes and discriminatory behaviour towards older people and further explored age-group differences using multi-group analysis. Data were collected from 1,500 Korean adults aged 20 and older in 2011 using the multi-stage quota sampling method. Trained investigators collected data with a developed survey questionnaire in person and a structural equation modelling method was used for data analysis. Attitudes towards older people was measured using two variables: ‘image of older people’ and ‘prejudice against older people’. The findings revealed that the younger and older generations demonstrated differences regarding the causal relationship between knowledge, attitudes and discriminatory behaviour towards older people. The paths model between independent and dependent variables in the structural equation modelling was non-equivalent in younger and older groups. For both groups, knowledge about ageing and older people led to a positive influence on their image of and prejudice against older people. The effect of prejudice on discriminatory behaviour was significant in the younger group, but not in the older group. Further implications for practice and future research to reduce ageism were discussed.