This is normal, because kittens rely on their owners to be with their owners often, so they purr more frequently. Cats that grow up are independent. They are not as dependent on their owners as they were when they were young, so they will not purr often. Of course, this is also related to the development of the cat's vocal cords. It may be that the voice becomes smaller as it matures.

Tip:

When a cat purrs, it usually means it is in a happy mood, and when it purrs, it warns you not to come near.