"I feel old," John studies his wrinkled hands and realizes that he wasn't just feeling old. He was old. The wrinkles that once weren't there were now very visible. When he closes his hands into a fist, it shakes. The strength it once had had disappeared. He wasn't as strong as he was before. 

If Sherlock asked him to punch him - like he did at that time when they were after Irene Adler - he probably wouldn't hurt the consulting detective. 

Sherlock looks at him from the other side of the sofa and says, "So?" 

John shrugs and slouches a little on his seat, "I don't know. I can't... I'm not... capable anymore."

The doctor stares at his hands longer... they weren't John Watson's hands anymore, he thought to himself. The John Watson that was a soldier, the one who had a steady hand and a firm grip, they weren't the hands of a crackshot. They weren't the hands of the John Watson who can protect Sherlock Holmes from bad guys. They can't punch and grip and be steady anymore. They're just a pair of weak hands now. 

He's not the John Watson... not the John Watson. He feels old, he feels weak, he feels more invalided than he was when he got shot.

He can't be reliable to Sherlock anymore - no, he's not really worried about being reliable (he can still make him tea and cook for him, can't he?), he's worried about not protecting Sherlock the way he did before.

Maybe he should find a new partner. Someone who's still strong. Sherlock should have done that a long time ago. He should have known this was going to happen.

"Too loud, John." Sherlock says.

John turns to him, "What's too loud?"

"You know what."

John glances at his hands again. Weak, old... and you know where 'weak and old' leads to.

"Sherlock?"

"Yes?"

"Have you ever thought about... separating? Death? Dying?" 

"Is this your way of breaking up with me? Bringing up your wrinkled hand and mentioning how old you feel? Because I'd rather you tell me the truth than talk shit."

There was a pause and it seemed to John like the world has stopped. Sherlock never spoke like that. 

Sherlock exhales and closes his eyes, "You're John Watson. You are still the John Watson - I don't even know what that means, but you are. You are, John. You're not weak, you're not old. You can still protect me. You're -- what you're thinking right now is pointless. I am not going to find another partner. I can never find anyone like you. I can never find another John Watson, because there's only one John Watson in my life, and there will only be one John Watson in this eart. There will never be another. I can never imagine myself living with another human being who is not you. I can never imagine solving crimes with another human being who is not you. I cannot imagine myself living without you, so shut your mouth, shut your thoughts, don't think about that anymore and just... just stop it, okay?" 

Sherlock's voice is shaking and so is his hands. His brows were knitted and he squeezed his eyes shut. He's not going to look at John... he decides not to.

John's heart was beating really fast - he didn't know if it was because he was nervous or guilty or because there was too much love in there and he could not stop them from overflowing at the moment. 

"Sherlock..."

"Stop. Don't talk." 

"Why?"

"Just shut up, John."

John shifted in his seat, now he was facing Sherlock. He wanted to take the detective's hand and hold it... just hold it... 

John was wrong for thinking that way. John was wrong for thinking that Sherlock is ever going to replace him or think less of him now that he can't do the things he did back then. He was wrong and he was sorry. 

"Sherlock."

Sherlock didn't speak nor look at him.

John moved closer to Sherlock and took the latter's hand in his. He kissed it and brought it to his chest. 

"I'm sorry," John said because he really was sorry. He meant it, he really did. He was wrong, very wrong for thinking that. He shouldn't have. 

"Shut up." Sherlock was holding back a sob. 

John kissed his hand again, "Sherlock, I really am sorry. I am sorry. I'm sorry..."

Sherlock faces the other way and says, "I hope you choke on your tea."

"I'm sorry..."

John keeps on kissing Sherlock's hand and Sherlock keeps on ignoring him and saying rude things but John knows he doesn't mean it - he understands that Sherlock was angry at him for bringing up 'separation'.

John keeps on kissing his hand, he will keep on kissing his hand until the detective forgives him... Sherlock squeezes his hand one time and that was it, it took quite a while but John knew he was forgiven. 

"Look at me, please?" John asks Sherlock, but the latter shook his head. 

John begged and Sherlock gave in. When the consulting detective faced John, his eyes were red and he was biting his lower lip to keep himself from sobbing. John's heart breaks at the sight of him and instead of talking about it, instead of apologizing again, John chooses to embrace Sherlock. 

He really did hurt him. Fuck. 

John whispers "I'm sorry" over and over again until Sherlock shushes him and asks him to shut up. 

"I've never thought about... separation," Sherlock said the last word with distaste, obviously he hated it and John was an arsehole for bringing that up.

Sherlock continued, "I've never even thought about us dying... I've never thought about endings even if I know that we will die someday. I've never thought about it. I've always thought about us running and chasing criminals even if we're both suffering from arthritis and backpains. I've always thought about us still solving crimes together, going to Scotland Yard together, judging other people together, bickering and fighting over petty things, laughing and giggling at crime scenes and at inappropriate times. I've never thought about death. I've never thought about any form of separation because... because that's... I don't want it, John. I don't want it. I need to get used to the thought, I know, but I don't want to get used to that yet. No, I want to be with you. I only want to think of all the things that we can still do now that we're still alive. That's it."

Sherlock was biting his lower lip to keep himself from crying and it was heartbreaking. Really heartbreaking. 

"I'm sorry..."

"I know and I forgive you."

"I love you."

"If you do then stop being such an asshole and stop thinking about bad things."

"Yes, I will. I'm sorry."

"Stop apologising!"

"Okay, yes."

John was speechless. He doesn't know what to say, except "I really am an arsehole." He really was an arsehole.

"I love you, you know." John said for the second time.

"I know." 

"And I want to be with you forever." 

"I do, too. So let's not think about death and separation."

"I want to solve crimes with you even in heaven."

"There are no crimes in heaven... and we don't even know if there's a thing called heaven... or if we could get in." 

"I want us to fight over petty things in heaven."

"Yes."

"I love you, Sherlock."

"I know."

"I love you."

"I love you."

John smiles and eases closer to Sherlock, cupping the detective's face with his hands.

"Forever then?" John smiles at Sherlock.

"I don't mind forever."

"I don't mind forever, too." 

"Because forever... that's what I want." Sherlock closes the distance between them and they kiss. 

 A/N:
I was listening to Growing Old With You and this... this just happened. It sort of happened. :> Hope you like it. Please feel free to point out the grammatical errors, I was too lazy to edit. Muwehehe.