That wouldn’t be object oriented. In OO you tend to want to ask an object about itself, Yegor talks a bit about this in his book Elegant Objects. What you are

That wouldn’t be object oriented. In OO you tend to want to ask an object about itself, Yegor talks a bit about this in his book Elegant Objects. What you are proposing is just functions or data-oriented programming; which is fine if that’s your thing, but I’d be weary because of the reasons you outline above. Can a book be an adult? What about a tv show? Or recipe from the 9th century? isAdult really only applies to users and really belongs on that object.