If you're looking for a fun and free app to determine your mental age, look no further than 精神年齢診断 - あなたの心はおいくつですかTestii. With over 650,000 downloads, this app by Testii has become a popular choice for those who want to know the gap between their real age and mental age.

The app is easy to use, with multiple simple questions and psychological tests that will diagnose your mental age and provide a detailed explanation of the results. Additionally, you can check a radar chart to see how well you fare in the five virtues that adults should wear. Whether you're interested in psychological testing, fortune-telling, or diagnosis, or just want to kill some time, this app is perfect for you. It even has a share button so you can share your results with friends on social media platforms like LINE, Twitter, and Facebook.