Background Studies investigating investment in health across the life course are lacking. The aim of this study was to examine investment in dental health across adolescence. Methods Changes in dental health investment, as measured by dental registration (months) between when adolescents were aged 11/12 years compared to when they were 15/16-years-old, were investigated using ordinary least squares (OLS) regressions. Adolescents aged 11 or 12 years in April 2003 in the Northern Ireland Longitudinal Study were included (n = 13,564). The overall change in registration and changes according to socio-economic status, highest educational attainment of household reference person, parental marital status, as well as the individuals' gender and number of siblings were examined. Within variable disparities at both age groups were also investigated. Results Average number of months registered with a dentist fell from 8.14 months (11/12 years old) to 7.38 months (15/16 years old) (p < 0.001). No gender disparities existed when adolescents were aged 11/12 years but when adolescents were 15/16 years old, females had significantly higher registration than males (8.72 months: 8.20 months; p < 0.001). Conclusions During the transition from childhood to adulthood, an individual's dental health may suffer as a result of a decline in registration rates with a dentist. This risk is likely to be greater among males than females. The role of children's services within dentistry should be reviewed.