I wouldn't think the average was 50 - some equines have been known to live into their 50s but it's pretty rare. This article says that minis average 25-35 years.
The reason I ask is because tomorrow I will be putting down Honey my appy/pony at the ripe old age of 41. I was just thinking about Christopher the 25+ mini and his old age. Although he's pretty healthy, just want to look forward to many more years with him. Thanks