In my experience insurance has never decreased with car age. They always try and push it up year on year.
Nothing to do with the age of the car, it's the length of time you've owned it that matters.
Of course, any reduction can be swallowed up by general premium increases, but for an identical car, it will be cheaper to insure one you've owned several years than one you just purchased. If you have your details on a comparison site, it's very easily proved by changing the length of time you've owned the car, or try getting a quote for an identical vehicle, same age, etc., but say you have only just bought it.
I've owned some cars 10-15 years, and by then, it makes a substantial difference. I've had this car since 2018, and I'm already paying about 15% less now than when I bought it, same insurer, always full NCD. It doesn't pay to keep swapping cars.