Miles 'Tails' Prower made his debut in 1992. In the video game his character in eight years of age. In the TV show Adventures of Sonic the Hedgehog, he is a much younger four and a half years of age.
He probably was supposed to have a twin, but they morphed together in their mother's womb except for their tails, therefore Tails possesses his tail an the tail of his sibling that never was....