Usually younger cats are just more energetic and playful in general. While some of this may seem like bullying to us, this may just be the way these two interact. If the other cat is continuing to eat, drink, and go to the litter box, then it should be fine just to let it go. I have one cat who occasionally runs into the room and pounces on her sleeping housemate. They tumble, the other runs off. Then it's like nothing happened. It just seems to be that some cats interact with others that way. What may seem weird to us is okay to the other cat.