20-22 years the the limit for a dimensional asphalt shingle roof in southern Illinois (which is not far and not that different of a climate from Louisville, KY). 18 years is when we start thinking about a new one. I was one of the last people to replace my roof in my subdivision, and it was well past its prime at 23 years... even though it was rated as a '50 year' shingle when it was installed.
A couple of moderate hail storms (which are typical in the midwest) over the span of 20 years, will rapidly age a roof. Add to that hail, the fact that we get it all.... Heat, cold and wind. One good hail storm will finish any asphalt shingle roof off, regardless of age. My boss is now on her second roof in less than 10 years.
My roof was the first insurance claim on my house in 23 years. I have replacement cost coverage, so it was covered at 100%. We'd had a widespread hail storm come through the previous summer, so lots of roofs of all ages were being replaced.
Starting about 5 years prior to the roof claim, my homeowner's insurance premium had been increasing by around 20% each year, with zero claims ever. I originally was not planning on claiming the roof on my insurance... but the insurance company had already been increasing my premium as if they were expecting a claim. So I decided to give them one.