Cats do spend less time actively playing as they get older. You could try encouraging them with some new and/or interactive toys. Different cats respond to different things - one of mine still loves chasing the laser pointer dot, especially when I run it across the couch so she can dive at ti full speed and just bounce when it escapes, the other has no interest in the "red bug" but really likes wrestling with length of thick string. One of them will bring me a small ball to throw for her. They both like hunting the feather wand.
Cats do spend less time actively playing as they become adults. Play is partly about learning what they can do, and once they get that figured out they don't need to spend as much time practicing or exploring. Again, giving them something new to investigate, or getting involved in playing with them, may be more interesting for them.
But in general, adult cats are very efficient creatures. When nothing needs to be done, they sleep... or find someplace to sit and watch birds or humans or other oddities.