We've had three horses pass in the two years I've worked at my barn. One was only 14, but had a tumor on her pituitary gland. She was getting free-choice alfalfa, soaked beetpulp 2x a day, ricebran, 6lbs sweetfeed, and still was a 2 out of 9 on the BCS chart. She would trot out to the pasture and back, so no one thought anything of it.
Monkey was suffering from a progressive neurological condition (from possum poop?) and he was 40. He couldn't walk anymore= put down.
Sugarbear, my trusty steed for two years of lessons carried me on my first (disasterous) jumping lesson. I was going over groundpoles, and he kept trying to take me over the 6"-1' jumps that were left out. His owner rode him over them to show me how it was done. The whole lesson was an hour, mosty trotting. The next night he passed.
It depends on each horse, but if she is still being spunky and energetic, then I'd let her chose.