К сожалению, сделать подобную, достоверную статистику, практически не возможно, так как бОльшую часть лошадей не вскрывают и причину смерти установить практически невозможно.

Будьте любезны, дайте ссылку на причины смерти лошадей после 30-ти лет. Это было было бы очень полезно.  Оообенно, интересны, "кишечниковые"....

Ту статью никак не могу найти, но в процессе поиска нашла очень на нее похожую, даже более длинную. Возможно, это другая перепечатка того же исследования в другом источнике, очень и очень похоже, только группы возрастные по другому указаны и нет лошадей за 40! Пони тут уже составляют не большую часть, а всего 1/3, но тоже неплохо:

Problems Affecting Older Horses

A report from the University of Kentucky Livestock Disease Diagnostic Center gives some valuable information about the disease conditions that affect older horses and the implications for the care and management of these horses (Disease Conditions in Geriatric Horses Equine Disease Quarterly, January 2000).
Horses studied were those presented to the Center for post mortem. Fifty four percent were 15-19 years of age, 33% were 20-24 years, 9% were 25-29 and 4% were 30 years and older. In the 30 years and older group, one third were ponies, while in the 15-19 year age group only 3% were ponies, suggesting greater longevity for pony breeds.
Many different conditions were diagnosed with the majority pertaining to gastrointestinal, musculoskeletal and reproductive systems as well as various types of neoplasia (cancers). Other less common diagnoses included diseases affecting the nervous system, liver, heart and lungs.
Gastrointestinal (bowel) problems were mainly displacements or twists with strangulation of some portion of the gastrointestinal tract. A common occurrence in these older horses was abdominal lipomas (fat tumors), a number of the cases with strangulation were the result of lipomas wrapping around the intestines.
Most of the conditions involving the reproductive system were cases of rupture of the uterine artery (the main blood supply to the uterus) and subsequent fatal haemorrhage. This problem occurred in mares at the time of foaling and was most common in the two younger age groups that were still reproductively active.
The musculoskeletal problems most commonly identified were fractures, laminitis, arthritis and other traumas (in that order). An increased incidence of cancer is associated with advanced age. In the two oldest groups, this was the most common individual diagnosis. The most common type of neoplasia found was pituitary adenoma (a type of brain tumour), followed by melanoma (a skin tumour most commonly found in grey horses), squamous cell carcinoma (a type of cancer affecting parts of the skin around the eye and reproductive tracts) and lymphoma (a generalised cancer affecting the lymphatic system).
In general, infectious diseases are an important cause of death in horses. However, infectious diseases were a much less common cause of death in older horses compared to conditions that were anatomical, traumatic or neoplastic in nature. The most common infectious problems were pneumonia and enteritis (bowel infection).
Considering the large number of gastrointestinal disorders, the researchers recommend that older horses should be fed a diet with ample high quality roughage, abundant water should be available and exercise provided. Older horses also need regular deworming and dental care. Stocking rates should be appropriate for the pasture so horses have access to sufficient feed. Older foaling mares should be closely monitored and veterinary assistance sought immediately if undue pain or signs of shock are observed.

Если бы у меня было на русском, я бы, естесственно, выложила на русском. Но такие материалы есть только на американских, британских, новозеландских, австралийских и др. сайтах. С английским у нас в стране все-таки лучше, чем с медициной. Поспрашивайте вокруг, наверняка кто-то из друзей сможет это вам перевести.