When Five is four, his powers are new and unwieldy. It's no surprise that he gets lost, but it is that a blue box appears in the alley, awash with the same blue energy that Five knows as his own. She tickles in the back of his head, warm and present and inviting. He asks her if they can be friends. She says yes.
She shows up again. And again. Five grows up with blue at his fingertips and the ability to jump through time and space. He grows up sneaking out and dodging the TARDIS's madman with his many faces, folding himself under a variety of consoles and talking for hours with his first and only friend.
He's twelve when he asks her if he she thinks he can time travel. He's twelve when she says yes, and shows him how.
He's thirteen when he leaps forward and something is terribly wrong and the blue energy twists and screams because time is wrong and he's jumped into a future that should not be. The energy might not obey but there's another way.
He calls, she answers.
The Doctor has been going to the same city for lifetimes. She isn't expecting an apocalypse that shouldn't be, and an impossible boy.
Looks like it's time for an adventure.