Harry really should have realized it after waking up in his cupboard and no longer being hungry. Instead, he just shrugged and walked out to eat something. He didn't notice that he'd died of starvation.
Harry really, really should have realized it after being bitten by the basilisk. Later on he found out that not even Phoenix tears could counteract the venom. At the time, however, he had no clue he'd just been poisoned.
He really, really, really should have realized it after fourth year. Voldemort had hit him with a spine breaking curse, but all it did was force him to bow. Voldemort's eyes widened, but the Dark Lord assumed it was just because he hadn't cast a spell in thirteen years.
Harry began to suspect it in his fifth year. A spell from Bellatrix LeStrange hit him a glancing blow and he fell to the ground. His head cracked open against the corner of a stone bleacher, leaving a large red smear dripping slowly. By the time he got to his feet the injury was gone.
Harry tacitly knew in his sixth year. Before Dumbledore conjured the ring of fire, an inferi had gotten to Harry. It wasted no time in biting out his throat. Harry didn't react and just kept fighting, throwing the inferi off of himself.
Harry admitted it to himself after the battle of Malfoy Manner. LeStrange had thrown two knives and one was buried in his chest. Harry pulled it out and laid it aside, choosing instead to cradle the bleeding elf in his lap.
Voldemort suspected before hitting Harry with the killing curse. He wasn't sure and had asked if the boy was dead. He knew after the boy rolled out of Hagrid's arms and into the battle. It made sense though. The blasted boy just wouldn't die.
Harry's friends suspected on his twenty seventh birthday as they looked at him and saw that he had not aged a day in the past ten years.
Hermione knew when she saw her best friend run through by a rogue Death Eater on an Auror mission. Shaking, she watched as Harry blinked before pulling out the large metal pole embedded in his stomach. There was no wound.
Ron knew after seeing Harry writhe in pain while poisoned. A bezoar passed his lips but had absolutely no effect. Twenty minutes later, Ron was staring at the still body of his best friend and honorary brother. Harry inhaled sharply and sat up, coughing like he was choking. The bezoar flew out from between his lips.
Ginny left when Harry was in his mid-forties, taking the kids with her. He didn't blame her. His own children looked older than he did at the present moment. That was the last time he saw his kids as kids.
Harry became frustrated when Luna died of old age. She was the last of his friends to go, hanging in there until her three hundred and fourteenth birthday. Harry looked at his wrists and sighed, knowing that nothing he did would help.
The merepeople kicked him out in his early 400's. Apparently Harry's presence living under the lake had become disruptive to the Giant Squid. Harry didn't really care, but he left anyway.
On Harry's 569th birthday he decided to go back to Hogwarts. Harry Evanson appeared as an exchange student to retake his fifth, six, and seventh years of school. No one heard from him after he graduated.
It's seven hundred years total before he hears the stories appearing in the now combined Wizarding and Muggle world about Harry Potter, the poor boy who'd sold his soul to save the Wizarding race. Harry decides that he quite likes the myth.
Three weeks after his 912th birthday, the world goes to hell. The muggles use their nuclear bombs, microwaving the world until almost nothing remains but the hidden cities, cockroaches, and Harry.
The hidden cities start leaving for other planets a while after Harry has stopped counting. He's passed the thousand year mark, he knows that, but he frankly can't bring himself to care about his age. Harry catches one of the last shuttles off the planet.
The new planet is a lovely place. A little disconcerting after the blues and greens of earth, but Harry quickly adapts to the orange sky with its twin suns. There are people here a bit like him in their relationship with Death. Unlike them, however, Harry decides to keep his birth name. No anonymous title for him.
Somewhere down the line something happened. The people of the planet seemed to live longer and longer, until the average lifespan reached into the two thousands. Harry decides to stay for a while, but even a species as long lived as them notices something off.
Language has changed greatly since Harry's day. His own name has come to have a complicated meaning of "tragic lost soul wanderer" in at least six hundred languages. The day he hears about time travel, though, he still can tell what's being said.
Funnily enough, it was the planet he spent all that time on that worked it out. Sleek capsules designed for watching, not interacting. Harry steals one immediately and returns to Earth. No rush though, he had plenty of time.
Camelot really is a lovely place, Harry decides. At least, now that that clotpole Arthur was King, Magic was allowed once again. Harry's wand had disintegrated a long, long time ago but he didn't need it anymore. Wandless and wordless magic was much more fun. As was disguising his time machine as a walking staff. He always did like the name Merlin.
When he starts to near his own time again, Harry calculates how old he is. He comes up with the number 12,384. Harry has to sit for a while after he realizes that.
The world marches on.
Harry skips the nuclear destruction. He has no wish to see that again. Instead he goes to a New Earth and sinks into a cave. Surrounded by crystals he sleeps, resting for years upon years. When he wakes he feels better, less drawn and tight, able to have fun again.
Harry explores the Universe, occasionally running into other travelers, but mostly on his own. He plays the good guy. The bad guy. Even the damsel in distress a few times, just for kicks. He creates new races and bolsters others. He causes and resolves conflicts, always staying in the shadows. A race he didn't create has come to worship him as a god. And time passes quickly.
The year is 561488901258097 when the Universe ends. There is no dramatic erasure of stars, or wails of sentient species, nor even a cosmic event that causes it. On a day in what Harry supposes should be around mid July, the Universe just gives a peculiar kind of shudder and then ceases to exist.
Harry walks on in nothing.
-
"For either must die at the hand of the other" could be interpreted to mean that only Voldemort can kill Harry.