The transition from adolescence to adulthood is a critical stage in the human lifespan during which the brain still undergoes substantial structural and functional change. The changing frequency composition of the resting state EEG reflects maturation of brain function. This study investigated (post)adolescent brain maturation captured by two independently but simultaneously recorded neuronal signals: EEG and fMRI. Data were collected in a 20 min eyes-open/eyes-closed resting state paradigm. EEG, fMRI-BOLD signal and EEG-BOLD correlations were compared between groups of adults, age 25 (n = 18), and adolescents, age 15 (n = 18). A typical developmental decrease of low-frequency EEG power was observed even at this late stage of brain maturation. Frequency and condition specific EEG-fMRI correlations proved robust for multiple brain regions. However, no consistent change in the EEG-BOLD correlations was identified that would correspond to the neuronal maturation captured by the EEG. This result indicates that the EEG-BOLD correlation measures a distinct aspect of neurophysiological activity that presumably matures earlier, since it is less sensitive to late maturation than the neuronal activity captured by low-frequency EEG.