A 92 year old friend had both knees replaced at age 89. Stairs were her nemesis during rehab, so her husband built her a long ramp to to door of their (rather low) travel trailer. Ex-UHaul ramp. Too high an entrance works against the ramp solution, you have to sort out your own slope vs stair height problems.
Except for the stairs, she gets around outside as well as she expected to. I think the knee replacement is usually a mobility improvement, but it works better for those younger and stronger (55 year old SIL had a huge improvement by getting rid of the pain). Expect about a year to get to best results.