They would have been 5 when they first died.
They were experimenting with different combinations of fast body and slow mind, when they ran off a cliff due to not having fast enough reflexes to react to the cliff.
Harry died. As Harry, Harry, and Harry pulled themselves back together, the boldest of them was sucked into Limbo, before being pulled back into the now fully reconstructed body.
As they conferenced between themselves, they realised what they had discovered. They were immortal: as one went to Limbo, the others would pull their physical body back into shape.
At the would-be-age of 6, they communally decided to slow down their brain to hibernation speed, perfectly timed to wake up at 11.
Dumbledore sat in his office, worrying about Harry Potter. Ever since he had realized that the boy had never shown up at the Dursley's, he had been organising search parties. In order to keep up morale of the searchers, and convince them that he hadn't died yet, he looked in the Hogwarts' Future Students book. What he saw alarmed him. Instead of the "Harry Potter" he was expecting, he saw "Harry / Harry / Harry / Harry". The sheer impossibility of this could and did overwhelm even his mind. He fainted.
A/N: Another small update. I promise that next chapter will be when he arrives at Hogwarts. And believe me, he will enter with a bang, possibly literally. Just so you know, if a Harry is in bold, they roll every letter possible, and if in italics, they speak with a growling noise in their throat.
Flames will be converted from thermal energy to pure energy and used to fuel the increased speed of the Harry's.
Oh, and before I forget again, I do not own Harry Potter.