Because decedents undergoing forensic autopsies would have behaved normally before death, prostate volume according to age group can be confirmed with forensic autopsy materials. The objectives of this study were to first confirm the current prostate volume by age and then determine diseases that can influence prostate volume using forensic autopsy materials. Prostate specimens were collected from forensic autopsies performed at Shiga University of Medical Science, Japan, between January 2015 and December 2019. Overall, 207 decedents were included in the study. Prostate volume was measured by the Archimedes’ principle. Concomitant diseases were determined by the past medical histories and autopsy results. The mean crude prostate volume was 29.1 ± 10.3 mL (range, 2.8–88.0 mL). The crude prostate volume increased with age. The mean corrected prostate volume (divided by body surface area) was significantly higher in patients with atherosclerosis than in those without. However, multiple regression analysis revealed that only age influenced the corrected prostate volume. Age was the only significant influencing factor for prostate volume. We propose applying age estimation using prostate volume for forensic medicine purposes. Because prostate volume was not influenced by concomitant disease, it would be valuable to estimate the decedent’s age using the prostate volume.