When the GIW became too much (too much for Danny, the ghosts and his family), everyone made the decision to close the portal. Danny wanted to stay, he wanted to experience all that was left of his life, but his family wouldn't let him. Jazz was terrified that the GIW would find out that there was still a ghost on earth, that they would hurt him, so at the last moment she pushed him out and yelled that they would let him come back when it was safe.
Years passed, years where the ghosts watched as the cheerful halfa faded, as he missed his family every day. As he aged and stopped aging (Clockwork explained it had to do with his feelings, Danny felt nothing); the halfa kept waiting, but nothing happened.
Then, a portal opened. Danny got excited, thinking Jazz would be on the other side but the world was not the same. He scanned the abandoned lab and found Vlad's face, he looked like an old man. Danny frowned and asked about his family, but Vlad denied sadly, then he explained that he had lived a long time (much longer than he should have) but he was starting to die.
Vlad also explained that Amity didn't exist anymore, that Sam, Tucker and Jazz had spent their lives disbanding the GIW, making the world a better place for him. But none of them stuck around to see the outcome; there were always rogue agents of the organization, always a danger to Danny that none of them wanted to take.
They made Vlad promise them that he would eliminate every piece of the old government agency (and he did, there wasn't even paper evidence left) and bring Danny back.
Years passed since then, and although Vlad doubted he had long to live, he kept his promise. Danny didn't take it very well, but he let Vlad speak, Vlad even took him to his family's graves. And above all, Vlad let him make a choice: He could keep the portal open, live in both worlds, or he could close it, stay on one side or the other.
Danny decided to stay, he did not close the portal but decided to live the human life that had been taken from him. He asked Vlad what he could do, and the former halfa shrugged his shoulders, he indicated that he had left him his fortune, so he could do what he wanted. He also warned him that "heroes" were beginning to be born into the world.
Not quite understanding the warning, Danny did what his clone had done in the past: travel. It felt strangely liberating to explore everywhere, to see how the world had changed. On one of his travels he stumbled upon a place: Gotham.
His obsession stirred as he saw all the crime in the city and he wandered into "Park Row" with a frown on his face. There he found a little boy, he looked like he was about to cry but was desperately looking for something. Danny decided to help him.
The boy saw him with distrust and asked him who he was, clearly he was not used to receive help; Danny noticed his blue eyes, his black hair and his fierce look. Then he looked at himself, he looked to be in his 20's, but they had similar features. An idea came to his mind.
So, thinking he couldn't lose anything, Danny said the stupidest thing he could think of at the moment.
"I'm you, from the future."
Jason was a little boy living a hard life, but the stars, the dreams and the desire to fly hadn't left his heart. When a stranger who looked like him told him he was his "future self" he believed him.
This, of course, changed many things.