Metabolism also slows with age, calorie requirements correspondingly drop and so should food intake really. There's a reason (healthy) seniors tend to be on the skinny side. Also something-something-three-score-and-ten.
and the healthier ones are similarly lean but more muscular.
also, while caloric needs drop, protein needs rise as older people show blunted anabolic responses to protein and thus need more protein to maximize muscle protein synthesis