The mean age across surveys was 19.7 (SD 2.9) in 1995, 21.6 (SD 3.6) in 1997, 24.2 (SD 3.9) in 2000, 26.4 (SD 3.7) in 2002, and 26.9 (SD 3.6) in 2004/2005. Due to the entry of a large new group of young participants, the mean age was 22.9 (SD 4.9) in 2009/2010. To study age-related changes in genetic influences on AAD symptoms, the data file was restructured from observations by survey to observations by age band. Six age groups were created: 15–17, 18–20, 21–23, 24–26, 27–29, and 30–32, since surveys were sent out every 2–3 years. If two or more observations of one individual fell into the same age band, the most recent measurement was selected (at which the co-twin also answered the CAGE). Table 2 shows the number of observations for each age group, separately for sex and zygosity.Table 2Number of observations for twins from (in)complete twin pairs per age and zygosity by sex groupMZMDZMMZFDZFDOSTotal no. of observationsComplete pairIncomplete pairComplete pairIncomplete pairComplete pairIncomplete pairComplete pairIncomplete pairComplete pairIncomplete pair15–1718618140103182616419290371,20818–20458104324858961584941407042773,64021–2345893288718441814981506122993,49424–263581161821087421964141455042743,03927–292321241521026182263121723103082,55630–321681291061025142292881662582692,229
