And you thought the Royal Commission into Aged Care disclosed bad treatment of old folk...

A story at the BBC (basically about the economics of looking after old people) starts with some extreme examples:

"I customarily killed old women. They all died, there by the big river. I didn't used to wait until they were completely dead to bury them. The women were afraid of me."

No wonder. That's the account of a man from the Aché, an indigenous tribe in eastern Paraguay, as told to anthropologists Kim Hill and Magdalena Hurtado.

He explained grandmothers helped with chores and babysitting but when they got too old to be useful, you couldn't be sentimental.

Brutally, the usual method was an axe to the head. For the old men, Aché custom dictated a different fate. They were sent away - and told never to return.  ....

As another anthropologist, Jared Diamond, points out, the Aché are hardly outliers. Among the Kualong, in Papua New Guinea, when a woman's husband died, it was her son's solemn duty to strangle her.

Update:  I see via Wikipedia that the practice is called "senicide", and it seems an entry that could have a lot more examples added to it, if the above story is anything to go by.  Most of their examples are from pretty ancient history, such as this one, notable again for its gruesomeness: 

The Heruli were a Germanic tribe during the Migration Period (about 400 to 800 CE). Procopius states in his work The Wars, that the Heruli placed the sick and elderly on a tall stack of wood and stabbed them to death before setting the pyre alight.^[7]

 Oh look, allegedly (there is no citation!) in Sardinia, the women got to be the "terminators":

An alleged custom was to throw incapable or ill elders off certain cliffs, a confirmed practice was the performing of euthanasia on ill, senile or suffering elders carried out by selected women named accabbadoras (lit. 'terminator' or 'ender') that after a blessing of the soon to be deceased would proceed to kill them through suffocation or blunt force to the back of the head by wooden mallet.