This is because older animals are more like senior citizens and can become confused easily, can be slower in their reactions, responses, or in their grasping of new concepts. Increasing intelligence could be compared to raising a baby. As they develop they are more responsive to different things. You have to expose them to the world bit by bit, introduce them to new toys, colors, tastes, and environments. This also includes socializing them to people of different ages, animals and many other things.
All animals are different.
Some will adapt better than others, which is why you should start this work at a young age. Any animal or child will tire of certain toys, while others will be cherished and carried around, slept with, or be looked for if removed. Specific bird toys are important for older birds and come in a wide variety of colors, shapes, and materials. Cats need toys that will keep them active and interested. Most of my client favorites are the scratch posts with the spring and a new toy attached periodically.