I heard a sports scientist on youtube say that a 70 year old male who lifted weights (properly, as in he knew what he was doing or was shown how to do it properly) could be as strong as your average 30 year old male.
Now no-one wants to be as strong as an average 30 year old male, but when you're 70, that's not a bad thing to be.
You can get strong at any age too; they've put 80 year olds on weight lifting programs and they've all gained strength.
There are also ways to work around dodgy hips and knees too.