Lots depends on where the car resides 90% of the time. Garage both at home and work or always out in the sun baking the oils out of the plastic. I do recommend the Novus products but would be using the #1 polish to restore the oils while if the plastic was old, yellow and gritty I'd be starting with #3 and then using #2 and finally #1. Do the inside of the window too. A good task for a contortionist.
They all get old. And the older the more brittle. A set of how to do it yourself articles re top replacement or even window replacement is
here.