In Sonic X, Tails met Sonic in the forest. Tails was getting bullied, and the bullies were breaking his small machines. As he was walking away, he saw a blue streak speed by him. He followed the streak, which was none other than Sonic the Hedgehog. Tails thought Sonic was really cool, and he wound up fixing up Sonic's red planne, the Tornado. Sonic said that he could use someone like Tails, so he agreed to let Tails come with him.
"There's the SonicX version, and then the Archie version. The Archie version is more dramatic, but the SonicX version, as Meph9 just explained, seems more probable to me. It relates to kids anyways, and in a mannor of speaking most the Mobian characters are roughly teenagers. Soo..."