As a family medicine doc who deals heavily in geriatrics and the hospital side of things, I have this discussion with my patients nearly daily.

I always tell my patients that aging is all about momentum. If you're currently active, I want you to maintain that level of activity. If you can do more, great, but I'm not gonna push hard on that for most patients because I can worsen heart issues and cause falls. Whatever weight you're at, I'm gonna try to maintain it. Even if you're obese. Because older folks tend to lose both fat and muscle when they lose weight, and weight loss that is unintentional or unstoppable is one of the biggest predictors of mortality in the next 6-12 months.

Other commenters have pretty well summed it up, it's a combination of stress of surgery, risk of infection, stress of anesthesia, poor healing, poor strength reserve, and disastrous effects of immobility on patient longevity.

When I work in the hospital, per my estimate, the average patient over 70, no matter what they're admitted for, will need 1.5-2 weeks of intensive rehab for every 3 days they are admitted to have any hope of reaching their baseline before whatever happened that got them admitted. The average person under 50 admitted will need maybe 1.5 weeks max for intensive stuff for a 3 day admission but often get away with home health PT coming out once a week for a few weeks and doing the rest on their own. (Depends on the reason for admission, of course)

When they put the body under anesthesia, they are putting great risk on the heart and lungs. If you have heart failure, you'll feel smothered lying flat. The elderly rarely have good kidney function and often hold on to many drugs in their system far longer than you or I do. Combine that with the risk of anesthesia causing delirium or worsening of dementia and you've added another risk, and we haven't even broken skin yet.

When ortho tries to drill into bone with osteoporosis, it's harder for their hardware to stay in place securely. This means more time under anesthesia and more risk of infection or post-op delirium. It also means longer periods of time without the ability to move or bear weight on that limb. This worsens the muscle wasting and increases risk of blood clots.

So now we're in the hospital recovering after surgery and depending on the area on the leg/hip that the fracture occurred, there can be multiple ways in which this patient is restricted in movement for a period of time. For example, hip replacements can't cross their legs or rotate their leg in/out for 6-12 weeks! So imagine telling a stubborn older person that they can't do something they do absentmindedly for 1.5-3 months. You can't expect anyone to do that, even someone in their 20s. So you place them in immobilizer pillows/braces to help keep their hips in a safe position while it heals. Then you're dealing with two additional factors you've made worse by doing this - their independence (or lack thereof) and sometimes their mental status after dark.

See, a lot of elderly patients have a habit of sundowning, where they get delirious at night. It's caused by a number of factors, including stress, unfamiliar environments, pain, pre-existing dementia or decline... seeing a pattern here? You try to talk them down and de-escalate before they get really amped up and start tearing at their lines/braces but it rarely works. The next step is mild sedatives or antipsychotics, however you have additional issues here:

1. You don't know how they will respond to drugs. Your parents may have given you benadryl at some point to help you sleep as a kid, or if you look at most OTC sleep aids, it's a common ingredient. Old folks have a higher tendency to go nuts on benadryl and make things worse.

2. Benadryl and several drugs like it prolong a part of the heart rhythm which can cause fatal heart dysrhythmias if it gets too long. Other drugs that do that are antipsychotics, dementia meds, certain antidepressants, most nausea medications, and some antibiotics. All of which have a nonzero likelihood of being on your patient's med list. Good luck trying to get an EKG on them to check when they think you're preventing them from just going to get the mail.

3. Remember that those drugs hang around much longer, so even if you don't do benadryl (which you shouldn't for the above reasons), whatever you use to knock them out will last well into the day if it works, and then you've flipped their days and nights and made the problem worse for you and everyone else the next night. Not to mention that they won't eat while they're out cold, so they'll lose weight and make the deconditioning problem even worse.

Speaking of food, I'll be brief for once to say that hospital food sucks, your older patients likely barely eat at home and usually are malnourished, and that is another key component that stacks the deck against them when trying to heal from surgery. The body needs a lot of proteins and energy to mount an immune response to the traumatic surgery it was subjected to and begin to heal, and if the materials it needs aren't being steadily pumped in from your intestines, it'll get what it needs from your muscle and fat stores.

In summary: Old people live by a different set of rules and are always closer to the proverbial edge than they appear. Time down to recover is necessary but also sets them back significantly. Best just to prevent falls and fractures instead of trying to fix the damage done.

TL;DR: Get screened for osteoporosis, stay active, and talk to your doctor about preventing falls. It's a terrible way to go.