This study aimed to explain why parental knowledge of adolescents’ whereabouts declines with age. Such an investigation is important because previous studies have established an association between behavior problems and low levels of parental knowledge. A time-sequential sample comprising 2415 adolescents aged 13–18 years was investigated on five annual occasions. Each year, parental knowledge declined by .10 SD. Adolescents’ establishment of a private sphere (less disclosure; defiance) was the most important mediator of age effects on knowledge. Taken together, declining parental control and the establishment of a private sphere explained 37.5% of the age-related decline in knowledge. Parental control was, however, not a significant predictor any longer when disclosure and defiance were controlled for. Results also revealed that some of the mediating variables were stronger in early-to-mid adolescence. Other variables appeared to slow the age-related decline, especially in mid-to-late adolescence. These variables are therefore interpreted as parents’ and adolescents’ attempts to balance autonomy development and connectedness. If this balancing fails, adolescent behavior problems might arise along with low levels of parental knowledge early on.