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quantumquandary
Since: Jun, 2017
SgtBenton
Since: Mar, 2022
Leee
Since: Jun, 2022

This was probably a short story in an anthology but may have been a short novel. I read it at school in the early 2000s in the UK.
The story is about these 2 or 3 teenage girls. They have a volunteer job keeping this elderly disabled lady company and looking after her home (over the summer holidays?) but they don't really do much and mostly just sit around gossiping and eating the lady's boxed chocolates - especially the cherry ones.
They previously had been "friends" with another girl who died falling from a cliff edge. They were false friends and it turns out that they were actually bullying her and caused her death.
The girls start feeling strange. They ache, their skin wrinkles, their hair whitens. Things get worse and worse with no clear cause and they bemoan that they do not deserve this.
The twist ending is that the elderly lady, having concluded from listening to the girls conversations that they are selfish, heartless and vain and that they caused the death of the other girl, has been poisoning the cherry chocolates. She got the poison as drugs from a doctor for her to take when she wants to euthanize herself. She tells them what they are experiencing is old age or death slowed down and that there's nothing they can do now to stop or reverse it.
It may have been called something like "a slow death". It was set in an English speaking country, possibly Australia.