Kastenman was born in Bälinge, Nyköping, Södermanland. His career began at the age of 17, when he enlisted at K1 in Stockholm. From there, he was transferred to Skövde in 1949. In 1969, his family moved to Borgunda, where he built a championship eventing course. During the 1970s and 1980s, he was the national coach for juniors, young riders and pony riders, and also coach of the Norwegian and the Finnish national teams. At one time, he coached five different teams. In his later years, he lived in a retirement home in Stenstorp, where he died on 10 June 2013 at the age of 88. His surviving next of kin, are his 2 children Björn and Mimmi Kastenman and has 5 grandsons and 1 great-granddaughter.