My little guy is 7 years old and he doesn't necessarily "suck", he constantly licks his left paw. I asked my vet and he told me it's a comfort thing. He only does this when he's lying on my chest before taking a nap. I guess we all have our little quirks...like rubbing our feet together before going to sleep, personally I twirl my hair...I gues our little guys have comfort quirks too!!! I wouldn't worry about it!