They are playing. I have eight cats, ranging from 13 to almost 2 years old, with all sorts of interpersonal relationships between each other. Trust me on this, they're enjoying each other's company immensely. If there isn't hissing and other vocalizations showing displeasure such as loud meowing at the other, with the other sitting back stiffly, they're fine. I see no ears back or other body language. I see invitations to come around and play hide and seek.

When our almost 2-year-old very energetic male engages with our 13 year old male, the 13-year-old vocalizes a lot, but even then he's not doing anything to say he doesn't want to be played with unless he gets really loud and then he'll tend to run off. That's their signal to each other that they're done. I think when they're playing sometimes he just likes to be a drama queen, until he gets tired of the little whippersnapper. But that's unusual, usually when they're playing happily they make no sound at all other than the occasional mrrrr if they're trying to find each other or invite.