The Life-stealer was an alien creature studied by the London branch of the Torchwood Institute, based at the Natural History Museum, in 1899 until it escaped on 17 May that year.
It possessed the ability to feed on youth, and so targeted younger Londoners starting with Archie, the leader of Torchwood London, resulting in them being rapidly aged while it was rejuvenated. However, it actively avoided elderly humans since interaction with them would produce the opposite effect, weakening the creature while giving the elder humans prolonged life.
It was pursued by Captain Jack Harkness and Queen Victoria, who realised the alien's weakness and ultimately stopped its rampage when she rallied a group of elderly Londoners to surround the creature, the forced contact with multiple people lacking youth reducing it to dust. (AUDIO: The Victorian Age)