At age 12 , the children who attended the clinic were more likely to be girls , have a higher birth weight , be from a higher social class , have older and taller mothers , and have mothers with higher levels of education compared with non - attenders . 17 There was a similar pattern of differences between clinic attenders who provided valid measures of physical activity and attenders who did not . The differences between attenders and non - attenders were similar at age 14 . However , the differences between children who did and did not provide valid accelerometer data were markedly reduced at age 14 , with the exception of mother 's level of education .