Thanks for your reply, Barbara. I think she was around 4 but could have been a bit older; she was full-grown when we got her.
She was always extremely healthy - she wasn't an inside chicken for health reasons. She actually decided on her own to live with us! We have a flock that free-ranges in our yard, and after a couple of years being a normal chicken, she started coming in and out the dog door into the house, all day long, and then at night, not going back in the henhouse with everyone else. We just ended up making a basket for her in the kitchen and she chose to live inside! (She still had access to the flock in the day and would spend some time with them each day).
She had so much personality. Would come running to the sound of my daughter's voice, would "cry" if left alone in a room, would hop on a lap to watch tv, loved to snuggle and cuddle. She was mainly attached to my daughter but was affectionate with all people.
My daughter is as attached to her as you would be to a beloved dog. This is going to be so hard for her. (Me too.)
I looked up the State Vet info and I really would like to know the reason, so maybe we will try to have her necropsied. I know I shouldn't dwell on it being my fault, but I feel like if only I hadn't put her outside yesterday... sigh. We have had many chickens die over the years but this one was like a family member. We even took her camping this past summer, lol. Heartbroken.