Yes, I have seen the suckling behavior in adult cats--although this might be a special case:

When we adopted Cherry (now in the Rainbow Bridge) in 1995, we discovered soon after that she was expecting kittens. She soon delivered four, beautiful, healthy babies, and needless to say, we adopted them all! In time, the kittens grew up and eventually were weaned--except for one of them...This cat, whom we named Honey, would continue to nuzzle against her mama throughout her adult life, trying to locate her mama's teats so she could suckle. Cherry of course didn't mind the attention--even if it was coming from her baby that by then was already as big as her!