"Mental Age" videos are all over TikTok right now. There's a website called "A Real Me" that has a free test you can take to see what your mental age is. It has nothing to do with how smart you are, but more about how mature/immature you are compared to other people your age.
It's not clear how scientific it all is, but people have been posting their results on TikTok. It's 30 questions and took me less than five minutes to complete. In the end, it tells you your mental age. You might be 36, but your mental age is 44, so you're a little more mature than other people. If your mental age is 26 and your 32, you're hip and young at heart. (That's totally not my score by the way)