I have been reading up on ages of various animals, and from wiki it takes about a couple that have lived well over 30 years, which I thought was the average life spam for a horse.

"Depending on breed, management and environment, the domestic horse today has a life expectancy of 25 to 30 years. It is uncommon, but a few horses live into their 40s, and, occasionally, beyond. The oldest verifiable record was "Old Billy," a horse that lived in the 19th century to the age of 62. In modern times, Sugar Puff, who had been listed in the Guinness Book of World Records as the world's oldest then-living pony, died at age 56."

Besides pollution, the first thing I could think of, what else has changed this life span to become so much shorter?

Just like a human, there are different factors that could play into life span. Genetics are probably high on the list. If the horse was feed properly, how hard they are worked, etc... If you don't take care of your body and burn the candle at both ends, your life span will be shortened.