No personal experience but a good friend had hip replacement surgery at 72 and knee replacement surgery at 74, last summer. She rebounded remarkably quickly and was playing tennis again (very slowly) within 4-6 weeks.
But she was fit to begin with, had no unusual reactions to surgery, and cooperated with the physical therapy. That makes a huge difference.
In contrast, my mom and grandfather were terrible patients where joint replacement surgeries were involved. Both were mostly inactive, and neither would do any exercise before the surgeries to prepare themselves. And both were very uncooperative with physical therapy. As a result my mom never regained range of motion from her first knee surgery in her 20s, so she limped and struggled to climb stairs or even get in and out of a vehicle. And my granddad's knee kept popping out of joint because his muscles and connective tissues had atrophied because he refused to do any exercise. I had to pick him up a few times after he fell, and he was a big man.
Worse, my mom had severe reactions to the anesthesia and morphine for pain, with hallucinations and disorientation that lasted for weeks. In a way that was a good thing. I stayed with her for months afterward to ensure she didn't have to stay in a nursing home, and did her physical therapy whether she liked it or not. She argued, cussed and screamed but she recovered better from her knee surgery at age 70 than the previous one in her 20s. Unfortunately she was less cooperative with her 2011 shoulder replacement surgery and still has trouble. She wants a joint replacement for her other shoulder but her doctor told her bluntly she wasn't a good candidate because of her age, reaction to anesthesia and morphine, and because she won't cooperate with physical therapy.
That's the real key. Staying as active as you can without endangering the surgery and new joint.
Personally I'd be cautious about riding an upright bike too quickly, but I'd seriously consider a recumbent at least for the recovery period.