Millie, the 31-year-old feline, has become the world's oldest cat through a diet of bottled water, salmon, prawns, chicken and tuna. Leslie Greenhough, 71, believes his tortoiseshell cat is the oldest living kitty in the world: Millie is 146 in human years. 'So they began dropping off Buxton water at my doorstep which I drank and also gave to Millie. Leslie said that it was his 'life ambition' to get her in the Guinness Book of Records as the world's oldest cat – but that he can't prove her age. 'She only drinks Buxton water and has a diet of salmon, prawns, chicken and tuna.