Do you see many police cars older than 10 years old?

I see lots of them. All of them retired from active service and often repainted or stripped of the police markings.

For what it is worth in the USA the standard for a patrol car is four to six years. Other administrative state (government) owned vehicles is nine years. So if a police car was used for administrative purposes it might remain in service long enough to be ten model years old.

By ten years most if not all have been retired and many are in the hands of civilians. In some states the markings are totally removed. Other states they are painted over.

One exception comes to mind and that is un-marked cars. But often a police chassis is avoided for undercover work. I say often because some of those who run drugs do like retire "P" optioned vehicles.