Our mounted police horses, some of those were carriage horses also, had a mandatory retirement at 15 years old or any time before that if considered unfit for service.
Our riding center had first dibs to those horses, maybe one every year or two.
Those made excellent school horses, half of them intermediate rider horses, not that many were beginner suitable.
We had some still giving lessons at 20+ years old.
My first jumping competition horse was one of those, assigned to me to show right off the police retirement and he was a handful still, but eventually became used to working less intensely.
Those horses don’t sit around calmly all the time, they are fit and well trained and ready for action in a split second.
I see why they would screen who would be a good home and that would not necessarily be some well meaning person that really is a beginner around horses.