Adult pilocytic astrocytomas (PAs) have been regarded as indistinguishable from pediatric PAs in terms of genome-wide expression and methylation patterns. It has been unclear whether adult PAs arise early in life and remain asymptomatic until adulthood, or whether they develop during adulthood. We sought to determine the age and origin of adult human PAs using two types of "marks" in the genomic DNA. First, we analyzed the DNA methylation patterns of adult and pediatric PAs to distinguish between PAs of different anatomic locations (n = 257 PA and control brain tissues). Second, we measured the concentration of nuclear bomb test-derived 14C in genomic DNA (n = 14 cases), which indicates the time point of the formation of human cell populations. Our data suggest that adult and pediatric PAs developing in the infratentorial brain are closely related and potentially develop from precursor cells early in life, whereas supratentorial PAs might show age and location-specific differences.