I'm still not sure the meaning of "I'm in good shape." My understanding is that I'm well physically. Can I use this sentence, talking about someone older? The sentence is like, "She is over 85 now, but she is still in good shape." What I mean is that she is very healthy and can do everything herself without any support.
You're correct - you can use it to describe an older person. The sentence that you have is one that a native English speaker would use.
"In good shape" is kind of relative to the age of the person you are talking about. When you say 'My girlfriend is in good shape" it usually means physically fit, but if you said "my grandmother is in good shape" it's more that she is healthy for her age.