Well, by adults, I pretty much mean like an Adult Sim. I suppose if you only see Elder Sims sparingly you wouldn't notice, but if you have a household you don't let age quickly or even at all at some point... Once an elder you'll see them often popping at the hip and out of nowhere shrug, look around him or herself lost asking something that sounds like "Suhgoya?" Perhaps if they do not have the active trait, most wouldn't care... But dang... If they get energized and autonomously decide to hit a treadmill...
I cringe. I cannot help but cringe, because once they are done you get to see their look of agony avatar and if you love the particular Sim, it gives a huge guilt trip, or you have the case where you purposely let them exercise and teleport them to the bathroom to take a bath or bubble bath immediately to ensure you don't see that, but the bleeding teleport fails and... yay... my Sim is in agony. Guilt trip + aggravation over the teleport.
Still, having known more adults and elders better than most age groups my entire life so far... Man, I know this is The Sims... but, I'd think if an elder Sim has the active trait, they would be able to stay active and not near kill themselves. The joint popping and the dementia-like actions, would be more like unknown traits that your Sims might have when he/she/whatever becomes an elder based on the traits they already have. Like the popping of the joints would be more common in lazy Sims and the dementia-like stuff would happen more often in Sims who don't exercise their mind a lot - don't read, research, do logical activities and such.
Or, perhaps, when your Sims becomes an elder, one could choose some sort of common habit or loss that comes from aging, like a sub-trait. Your Sim has aged up to an elder! New "traits" you can add: Hard of Hearing, Blind as a Bat, Wields a Cane, Steers a Walker, Fearless Flatulence, Champion Complainer (Grump who misses the good old days), Like Aged Wine (wise as heck), Pops like Popcorn (arthritis), Lost Time (dementia), Rocking Chair Marathoner (loves rocking chairs like how most Sims love molding clay)... I'm sure those Simmers I've seen talk about retirement homes would love that sort of variety in their residents.
Dang... if this cannot be changed, I hope someone might get inspired to make a mod like this...
For now, my active elder is actually aged as just a Sim Adult... It saves the Sim and me a lot of agony, but it is still annoying that I have to make them young again just so they act more like the elder they are based after as opposed to the stereotyped caricature I'm stuck with otherwise.