The word infanticide means:
killing of babies: the practice of killing newborn babies
Is there an equivalent term for killing the elderly?
The word you are looking for is one of either:
- senicide — as in senior, senile, senectitude, senescence, senate, senator
- geronticide — as in gerontocracy, ˌgerontologist, gerontophilia, gerontophobe
Senicide or geronticide is the abandonment to death, suicide, or killing of the elderly.
Apparently senicide was preferred until the middle part of the past century, but now geronticide has caught up and surpassed it, being now preferred by about 3 to 1.
See this Google Ngram: