One of my male cats does that he's 3 years old and he's been doing that since he was a kitten. It eases them to do that, they go back to their days of kneading on mama for milk. I don't stop mine.
There's a name for this condition - technically it's called pica. I've had various cats over many years do things like this. I don't think it's a big issue unless they actually start eating what they're suckling on. I had a Siamese who wanted to EAT wool. She managed to eat almost an entire glove before I realized it. Ugh!