From what all I've been told, minis live upwards of 30-40 years, so I do not consider them old in their teens, to be sure.
I hear this age estimate a lot, but have not honestly heard of a single registered Miniature living to 40. I know there are a couple forum celebrities like Gramma Pony in their 30's, but they do seem to be the exception. I would like to see some real research put behind this, as I feel a life expectancy of 30-40 is misleading.
We have retired many of our stallions and mares here, and old age is the most common cause of death, usually in their early to mid-twenties. Most of them were so healthy that their death surprised me despite their age, and it never gets easier. We just lost a 24 year old mare last fall--another one, thank God, that went in her sleep. Right now we have several middle-aged horses, two retired girls in their late teens (but not retired due to age), and our oldest is a 20 year old stallion. He is as healthy as he was at 10, and I am hoping to see him past the 30-year mark.
We have always done limited breeding, and of course that extra time and space makes it much easier to accomodate the seniors, but I can see how profit-based operations would have difficulty finding a place for their older horses. I have bought mares in their late teens before, and would again, as one nice foal is all it takes to replace them, and then they can go out with my other special ladies.
That said, some of the retired mares are not happy retired. The oldest mini mare that ever foaled here was 24. There was a stallion adjacent to one of the pasture fences, and she must've backed up to it. We had no idea she was bred--one of our farm hands thought she had a stomach tumor. Surprise.
Needless to say, we moved the stallion.