To better assess the pathology of neurodegenerative disorders and to evaluate the efficacy of neuroprotective interventions, it is required to develop biomarkers that can accurately capture age-related biological changes in the human brain. Brain serotonin 2A receptors (5-HT2AR) show a particularly profound age-related decline and are also widely reduced in, e.g., Alzheimer’s disease. Hence, cerebral 5-HT2AR binding measured in vivo using positron emission tomography (PET) is a potentially useful biomarker for age-related changes in the brain.In this study, we investigate the decline in 5-HT2AR binding to evaluate its usefulness as a biomarker for biological aging. Specifically, we aim to 1) predict brain age using 5-HT2AR binding outcomes, 2) compare 5-HT2AR-based predictions of brain age to predictions based on gray matter (GM) volume, as determined with structural magnetic resonance imaging (MRI), and 3) investigate whether combining 5-HT2AR and GM volume data improves prediction. We used PET and MR images from 209 healthy individuals aged between 18 and 85 years (mean=38, std=18). 5-HT2AR binding and GM volume were calculated for 14 cortical and subcortical regions. Different machine learning algorithms were used to predict age based on 5-HT2AR binding, GM volume, and the combined measures. The mean absolute error (MAE) and a cross-validation approach were used for evaluation and model comparison.We find that both the cerebral 5-HT2AR binding (mean MAE=6.63 years, std=0.78 years) and GM volume (mean MAE=7.76 years, std=0.92 years) predict chronological age accurately. Combining the two measures improves the prediction further (mean MAE=5.93 years, std=0.82). We conclude that when it comes to predicting age, in vivo measurements of the cerebral 5-HT2AR binding are more informative than GM volumes.