It's true that cats, like humans, need less food as they get older.
I adopted a lovely stray cat from the shelter- she was sleek and about a year old. She ate only dry food, and was very active. Over the next couple of years she slowly gained weight until she was obviously overweight. I had to cut down on her dry food drastically to make her lose weight- which eventually she did and now is an ideal weight again. This was all on dry food with no snacks!