I let both of my oldsters go last winter. The oldest, Lucky, had been mine and my sisters since he was 2, he was 32 when I let him go. He'd been getting more and more crippled up with arthritis and when the weather turned really cold, he got to where he was just ....pitiful. We had a blizzard coming and I knew he'd be really painful and miserable so a day before when it was still sunny and warm and he was running with the herd and feeling good, I took him in late in the day. It killed me and year later I'm still crying when I talk about it, but it was the right thing.
The other mare was old but 'only' 26. She had laminitis and had foundered in the past and her right front sole was so thin you could feel the coffin bone and we pretty much expected it to drop any day. Then I went out in the pasture and saw she'd gotten kicked or ???, I don't know, never found what caused the injury. She had opened her left front cannon all the way to the bone and couldn't walk on it. The wound could have been stichted up and cleaned up and probably been ok. The problem was, with the stress on the right foot, sooner or later that coffin bone would have dropped and come through the sole. So.....kill her now or kill her later? became the question. I shot her full of banamine so she felt no pain and could walk on both fores and took her to the vet. She trotted in like the park horse she was and held her head up high and proud. Again, the emotional pain was awful, but it was still the right thing to do.
Honestly, I hope to have all my horses to ripe old ages so that I have the time to decide when it's right for them to go, not lose them to disease or accidents too early.