Family theory has suggested a radical transformation of intimacy in large parts of the Western world over the last 50 years. Given this development, how can we best explain union form in older people’s relationships: In terms of the traditional values they were brought up with (cohort)? In terms of the historical context in which the relationships were initiated (period)? Or in terms of the life-phase in which the relationships were initiated (age)? All of these hypothesis have been suggested by prior research. In this paper we discuss these respective arguments, basing ourselves on results from a quantitative survey of 60–90 year old Swedes (response rate 42%), focusing a subset of respondents (n=702) who are currently either married, cohabiting or LAT. A logistic regression showed no significant support for the cohort hypothesis. It gives strong support for the historical hypothesis – union form is significantly explained by the year the relationship was initiated. It also gives significant support to the life phase hypothesis – older people tend to prefer LAT relationships – but only after the historical normative context allow it. The results are discussed in relation to explanations suggested by earlier research regarding union form in older couples.