Age and histologic grade are interrelated characteristics of diffuse fibrillary astrocytomas, because the peak age incidence rises with increasing grade. The relationship between age and grade may be explained if age determines the rate of anaplastic progression in astrocytomas. The authors tested this hypothesis by determining the interval between diagnosis of low-grade astrocytoma and progression to high-grade astrocytoma in patients of various ages. A two-way scatterplot of age at initial diagnosis versus interval to anaplastic progression demonstrated a strong negative correlation (n = 24; Pearson correlation coefficient = −0.83; Spearman correlation coefficient = −0.79; p < 0.001 for both values). It was concluded that the rate of anaplastic progression in low-grade astrocytoma is directly correlated with patient age.