Because he's either actually fourteen or something weird is going on.
Gon was twelve when he became a hunter.
He was twelve when he completed Greed Island.
He was also twelve when his revenge against Pitou almost became his downfall.
He had been twelve for far too long.
It was subtle at first, no one noticed for years. But puberty was supposed to hit at some point, and when he reached seventeen and no voice had deepened, nor any height grown, it became worrying.
Of course, the one affected never paid it much mind, too engrossed in his travels and adventures to worry about something so insignificant as his age – or lack thereof. And so, his friends grew older without him, wiser, stronger, more serious, more bitter.
And he remained a child of twelve.
It never really hit him until he searched for Ging once again, decades later, and found a strong, irresponsible, playful and rather old man.
It was then that he truly became aware of his situation.
How much time had passed? How many years? Decades? When had he started to look up at Killua because the assassin had become too tall? When had he started to call Leorio an old man in all honesty? When had he started seeing a rage filled, vengeance driven Kurapika every day as normal?
Was Mito-san still alive?
It had been so gradual and he never paid much attention to details, so he hadn't noticed. His friends were old. Not just adults, but truly, terrifyingly, greyingly old. The energy was gone from their bones, the familiar shine innocence or glint of mischievousness awash from their dulling eyes.
And Gon could only observe as they became even older.
Still a child of twelve.
The first one to go was Kurapika, in a blaze of rage and blood and an age old recklessness that had long since been imbued into his very essence. Gon had cried for a day and a night in the arms of his best friend.
The second was Leorio, assassinated in the veil of darkness of a moonless night for driving too many greedy doctors with connections out of their jobs. Gon had sat in the corner as he blankly watched his friend drown his sorrows in liquor.
The third was Killua, driven half-mad by grief and blinded by revenge, with the his sister's blood dripping down his face. Gon had heard something like glass shattering coming from inside him.
Broken from grief, he had finally deemed it appropriate to listen to those around him. And he heard the rumours.
The ageless one.
Immortal.
Youth incarnate.
Nothing but rumours and things he couldn't validate, couldn't look into, couldn't find out why.
And tired, oh so tired of this world, he hid within the forest that loved him so and went into a deep slumber.
Days.
Months.
Years.
Decades.
Centuries?
Until one day he opened his eyes to meet with the brilliant gaze of a curious boy.
And they played, for how long he didn't know, but they played and had more fun that he'd ever had since...
Since when?
There was nothing before this, was there?
His innocence restored and his past erased – though he would never know it – Gon enjoyed his life with his new companion.
Still a child of twelve.