I think I agree with her aging in cat years but not having the same life expectancy of Earth cats because she is an alien one, perhaps?
With her being the same age as Usagi currently, Luna is either in her 90's or was given to Princess Serenity in the past and put in cryo when she was just 2 years old, considering Serenity herself died at 14.
Of course, Luna may be very smart though considering she's an alien cat but I think she had to have many years of experience and exposure to learn all that she knows.