This research note examines birth cohort trends and differentials in age at first marriage based on the sample data of the 1980 Population and Housing Census of Thailand. Previous studies have compared the singulate mean age at marriage (SMAM) obtained from censuses and surveys in order to describe changes in the timing of marriage in Thailand. There are, however, some limitations in the use of SMAM towards this end. Some observations about differences in the age at marriage for different regions, urban/rural contexts, and religious and educational groups are also offered.