In June 2007, in a village in Begusarai district (Bihar, India), police detained a boy of about 8 for the death of a six-month-old baby. According to press reports, he admitted strangling her and led villagers to where he had buried her.
Police also looked into the earlier deaths of his baby sister and a baby cousin, which is why he is credited with three victims, all infants. The press called him “the world’s youngest serial killer”. That label is a media one: younger children have killed, but only once.
His age isn’t even settled. Most accounts, including The Guardian as cited on Wikipedia, say 8; Gulf News said 10 at the time.
He was never tried. Indian law doesn’t allow a child that young to be jailed, so he was sent to a juvenile home in Munger. Anything said about his life after that comes from sensationalist sites, so I’m leaving it out.
The case sits on the lists of youngest killers next to Mary Bell or Thompson and Venables in the UK, and those lists mostly show how much the age of criminal responsibility varies from country to country.