Before you read, please note that I am also unsatisfied with the androids' weight, too heavy for blending in with other humans. So I made some immersive changes.
Additional Note: Very detached from NieR: Automata, the game is not light-hearted enough. I want my story to be more peaceful and cheerful. NO LEMON!
-YNND

Chapter 0 Prologue
This chapter explains why 2B and 9S soon seems to become older, and explains 2B's later interest in human culture.

4 days ago
9S, after reconstructing his own body, spent 2 days hacking into himself, giving himself maintenance.
'9S, 2B, open a closed channel, I have a few words.' the commander radioed in.
'Well, 9S, you heard it.' 2B said, as she switched off her comms.
9S, hearing the message, was puzzled by what message the commander needs to tell him, the commander seldom requires him to use a closed channel. The scanner unit switched on their closed channel.
'2B, 9S, the humans are coming back in four months, just awoken from stasis. I want you to clear the last anti-human machine lifeforms out there. And for the main reason of the closed channel, I want you two to continue looking for A2, and either kill her or apprehend her.'
Command cut the conversation, and 2B, along with 9S, set out to eradicate the last hostile machine.

2B stands back up from a bench in the resistance camp, just after breaking it.
'Nine…uh…S, my chassis is too heavy.'
'Mine also…' said 9S, embarrassed.
Both 2B and 9S is wearing the Heavy Armour, which increased their weight a lot.
2B sat down on a rigid crate nearby. and picked up an item that was used by Resistance.
'Pod, what is this? A weapon?'
'This item is a weapon, this type of weapon was once carried by soldiers of humanity, capable of firing projectiles repeatedly. This particular model originates 9,998 years ago in the former human nation known as the Soviet Union. It was once popular among human militaries. This model is presumed to be the most successful one ever built.'
2B was fascinated by it, and tried to figure out a way to operate it.
'I mean, I always see you pods using them, but i seldom see a human operable one.'
'2B, I see you are finally interested in this. I also heard that the humans are returning, right? Glad to finally see that happen. But first, we have to clear the remaining hostile machines first. Wanna give this weapon a try?'
'Against machines?' 2B asked, 'I don't know i can operate it well.'
'Well, there are some targets there, you can give the weapon a try.'
*Gunshot sound*
'What was that?' exclaimed 2B
She turned her head to see 9S holding the same type of weapon, operating it, against some fake targets. He pulled on the trigger again and again, firing shot after shot. The weapon's recoil pushed against him slightly, but he could handle it pretty well.
'9S, is that the weapon? The one used by the human armies?'
'Yep. Impressive piece of weapon. Try it out, it is awesome.'
2B stood up, and walked to the 'Range' and picked up the gun.
A few hours later, 9S and 2B are out in the ruined city, still carrying swords. Darkness arrive, as they finished destroying a group of hostile machines. The sky turned dark, and the two androids sat down by the street. Suddenly a familiar figure appeared. It appears to be an android, with white long hair. It was A2.
'I have overheard your convocation with command, 9S.'
'What do you want, A2?' said 9S in disgust.
'YorHa Unit 2 type A, command ordered you be apprehended.'
'Seeing humans finally return proved i am wrong. I thought the humans are all extinct. I wish to apologise. I can offer you something, but you two must get my records clean from command.' She said, as she hands them a small box.
'Open them when the humans arrive. I found them in the alien's ship. It was meant to terraform the Earth, do not ask me how.'
'Wow…thanks, i guess. I will come up with an excuse for your supposed desertion.' said 9S.
'Yo, look, Androids!' said a person dressed in black armour, talking to his partner.
'Who are you?' 2B asked with a defensive attitude, holding her sword forward.
'Yo yo yo…chill, we are human pathfinders, military units sent down to look out. My name is Stephen Chen, just call me Stephen, and he is Andrew.'
It was a moment of relief, as they finally saw a human again, or two humans. 9S notices the two soldiers are wearing gas masks, as well as hoses linking to an oxygen tank behind. They are carrying a similar type of weapon they had seen used by the resistance, but it was lined with illuminating blue lights, along with their armour.
'What's with the masks though? Air is clean and good.' said 9S.
'Precautions, we have been in stasis for thousands of years, we don't know what earth had been up to, you know?' said Andrew.
'Well, the resistance camp is down the street, you will find more androids there. Good luck.' said A2.
They parted ways, A2 walked into the shadows of the night, while 2B and 9S returned to their mission.
On the way, they found more and more pathfinders, some fighting machines, some taking photos. Their behaviour is extremely from androids, from what 2B saw. They showed emotion, they joke, laugh and enthusiasm. They also showed flexible combat styles, using their ranged combat capabilities to make up for their weaker body.
Soon, 2B radioed the commander.
'Commander, I have reports of YorHa unit 2 type A.'
'Have you apprehended her yet?'
'Negative, unable to preform task. Unit A2 was…um…found to be under the effect of a virus, which caused her desertion. 9S hacked her and removed her virus, returning her back to normal.' 2B tried to come up with a fake explanation.
A2 walked to their side, she has been listening the conversation.
'Can you confirm 9S? Is A2 here?'
9S speaks confidently, 'Yes commander. I removed A2's virus.'
'Commander, i am fine.' Said A2.
the conversation ends formally. Moments later, A2's wanted warrant disappeared.

1 month later, in the bunker
'The humans are returning, and our units have been meeting up with human pathfinder teams. In order to prevent a virus from sabotaging our systems again and increase effectiveness, we will now be implementing a new system, and introduce a new operational chassis for all units.' The commander said, 'It will be lighter, and more closely resettle humans, as well as more modular. That means each unit can now perform different tasks, without requiring any extensive mods. Each unit can also choose their own appearance. For the purpose of closely monitoring the society, observer units will be equipped with an Age growth system, but breathing, eating and drinking is not required, but optional.'
'Commander, why the Age growth system?' asked 9S.
'So that the humans will see us as also humans. Ok, here are the units i will be sending as observers. First, 2B and 9S, A2 and operator 6O and 21O, as well as combat team 5. Your ageing system only changes your appearance, but you can always slow it down, the humans will find it strange if you stayed the same appearance for 10 years, without what they refer to as "Growing up", so it is needed. You are expected to return for maintenance checks every 2 years.'
'Hey 2B.' 9S whispered.
'What, 9S?'
'Do you think the age stuff is unnecessary?'
'No, I found it more closely resemble human anatomy.' 2B replied, expertly hiding her doubt.
'Fine.'
A few hours later, they are changed to new bodies. 2B and 9S chose to keep their original appearance, although 9S adopted a more adult look. And soon, they boarded their flight units.
'Lightweight chassis is good, but age? Really?' thought 2B, as she and 9S descended back to earth.
Cannon fire appears while they descend, signifying the final step for the liberation of Earth had begun.
[Chapter 0 end]

A/N (Author's Remarks):
Nothing much in this chapter, it serves as an explaining of why 2B, 9S and the other androids can age, and how they can live normally, as they have 'lightweight immersive chassis' now.
I made that part of the story and the name up myself.
Also, wooden benches are too fragile to accommodate 2B's weight along with her Heavy Armour, and guns are cool, specially AKs, I love Kalashnikov rifles.
The AK is meant to be the one Anemone is holding in her concept art.

-Your Neighbour Next Door