To study personality characteristics and behaviour problems in different age groups more precisely, the parents of 105 children with Williams syndrome (WS) were asked to complete a questionnaire with a list of 25 personality adjectives and 18 behaviour disturbances. Three age groups – children under 10 years, adolescents between 10 and 20 years, and adults over 20 years of age – were compared. Adults with WS were described as being less lively, determined, active, restless, tearful, quarrelsome, impertinent, and over-friendly in comparison with children with WS. Additionally, adolescents and adults were assessed as being better balanced and more withdrawn than children with WS. Females were found to be less cheerful and happy as well as more tearful and quarrelsome than males, but these results showed only a statistical tendency. A discriminant analysis was performed to prove whether the three age groups could be discriminated on the basis of personality aspects. The results showed correct classification to one of the three age groups in 86% of the individuals with WS. The most discriminating adjectives were active, lively, well balanced, withdrawn, being over-friendly, and vigorous. No differences regarding age or sex were found after calculating a composite score of behaviour problems reported in each individual. However, a comparison of single behaviour problems showed a decrease in external aggressive behaviours and greater depressive symptoms with increasing age.