The sagging posture and the holding their back is just an elders walkstyle thing. They do it less if their fitness is high or they have the active trait, but the only ones I've had ignore the sagging completely were the ones that completed the body builder aspiration - and even they will occasionally do the "my back" gesture. (also it seems more pronounced on female sims than male sims.)
Elder sims do have a faster decay on their energy need, but a slower decay on their hunger need, so that may also be a factor. At least I think they do. Since I play on long life, my elders frequently have the "seldom Sleepy" and "hardly hungry" traits from the rewards store, since they have a bunch of aspiration points available. So I don't notice need decay differences as much on them.
