The two I had euthanased were only two years old, but one of them hadn't laid an egg for nearly a year, and the other one was at the stage of laying wind eggs, membrane only, weird shaped eggs etc. They sat quietly in their carrying boxes in the car, and showed no signs of stress on the journey or in the vets surgery. I had kept on the non-laying bird for a year because the two of them were best friends, but once the other one had clearly reached the end of her lay I decided it was best for them both to go together so I can now revitalise my little flock with some new birds. I've found that, once hybrids get the stage of laying peculiar eggs, they will only go on and become increasingly at risk of degenerative diseases, probably painful, such as prolapse, so I call time whilst they are still on their feet and enjoying life reasonably well, (which is what i would hope for myself.) With room to keep only 4-5 chickens, eggs are important to me, and although they are much-loved pets the hybrids have to earn their keep. I give them very good lives whilst they are productive, and a peaceful end in return for all those lovely eggs.
Mind you, I'm sure I shall make an exception for Marigold and Nutmeg when they're older, having raised them from chicks. More personality there somehow!