IMPORTANT! This has some spoilers as it follows some of the newer things in the manga. It has Sabo but I'm pretending that Sabo's father never found him and the fire was never started so Luffy, Ace, and Sabo stayed together through their teen years.

Background info! Sabo is Luffy and Ace's friend who was actually part of their pact to become brother's, he knew Ace before Luffy even showed up and was living in Gray Terminal- a desolate place filled with people who don't care if they live or die anymore- where Ace would sneak away to see (they were collecting treasure together so they could buy a pirate ship). Luffy would attempt to follow Ace to Gray Terminal and failed a number of times before he finally made it and was tied to a tree by Ace and Sabo. Luffy was later tortured by a pirate for information on the location of Ace's and Sabo's treasure. Luffy didn't give any info away and was saved by Ace and Sabo, they all became friends and later brothers. Dadan is a mountain bandit who Garp is blackmailing to take care of Ace and later Luffy.

I don't own One Piece

Phase

When the Straw-Hats think about it they realize that no matter how powerful their captain is, no matter how childish he acts, and no matter how terrifying he can be at times he's still a teenager. Which brought the realization that Luffy had previously been a child, and if he was a child before, a teen now, and without the… Awkwardness of being a teen then Luffy had already gone through the horrible torment that was puberty. Which raised a question or two, the one at the forefront though was 'What was Luffy like during those years?'And if you were to ask those that knew the rubbery ball of energy at that time you'd get different reactions. Sabo's would be to scowl, Garp's to laugh, Dadan's to sigh, and Ace's would be to blush. For they had all been there for that time in Luffy's life, albeit they all had different memories when asked what Luffy was like going through puberty.

Growth Spurt:

It was Sabo who noticed it at first. And when he did he was not happy, Monkey D. Luffy, the little brat that had followed Ace all the way to Gray Terminal, the kid that had taken a vicious beating from Porschemy so Ace and him would be his friends, the child that Sabo considered to be his brother along with Ace, was now taller then him! And damn it! That wasn't cool! Luffy was his little brother, he was not supposed to grow taller then him!

When Sabo brought his plight to Ace the freckled boy had laughed and told him that Luffy had just hit a growth spurt and after Sabo hit one of his own did he would once again be taller then the rubber child.

And Ace was right. Sabo did hit a growth spurt, right before Luffy hit another one, which made the smiling child taller then him again! When the same thing kept happening Sabo finally gave up on being taller then Luffy, he did it with a scowl and a huff of 'He's doing it on purpose, I know it!'

Voice Cracking:

When Garp visited after being away for who knows how long he expected to find both Ace and Luffy how he left them, Ace with a frown adorning his freckled face and an aura of loathing towards the world in general surrounding him and a scrawny Luffy with the disposition of a child on a permanent sugar high.

But, what he expected to find and what he actually found clashed horribly. Instead of a frowning Ace with an aura foul enough to get tigers and bears to back down he found a smiling teen with scratches and other wounds abound, the aura that had hung around the freckled teen had all but disappeared. And Luffy, Luffy who was so child like with the barest hint of baby fat still lingering at his cheeks had grown up into a slender teen with the same smile. But what really got him, what really surprised Garp was when Luffy had yelled at him for giving Ace a punch filled with his love Luffy's voice had cracked! Right in the middle of his shout his voice had cracked going from his usual child like voice to a shrill high pitched keen for a matter of moments.

At the sound Garp had froze forgetting all about Ace in favor of staring agog at his other grandson whose voice had just cracked. And he continued to stare at Luffy until the straw hat wearing teen began to fidget under his gaze, at that point, after Ace had started to glare at him for making Luffy uncomfortable Garp burst out laughing.

Moodiness:

To Dadan the few years that Luffy went through puberty was a living hell.

It started with Sabo whining about Luffy being taller to Garp and the brat getting into a all out brawl in her house after the marine had laughed at Luffy's voice cracking and the straw that broke the camels back was when Luffy finally started to get moody.

The hyperactive boy was a handful already without his moods going haywire. The rubber child already aggravated alligators on a regular basis, fought with cutthroat criminals in Gray Terminal, and got dangerously close to the ocean for a fruit user of his age. All these things had brought the 'Wrath of Garp' down on Dadan's poor head, which Dadan thought was terribly unfair because Garp did more damage to the boy when he visited then Luffy obtained through his 'adventures'.

Luffy only got worse though when his mood went haywire, something that Dadan had thought previously impossible. Now the boy would be his usual happy go lucky smiles a lot self one minute then a dangerous and hateful demon the next. Dadan had lost count how many times Ace had come back to the house with Luffy in tow knocked out because the smiling teen had switched moods and went on a rampage till he wore himself out. On those days Dadan would see smoke rising in the distance and sigh, she was getting to old for this shit. As the smoke disappeared and Dadan got ready for the moody teen to wake up so she could knock him out again she'd always promise herself the same thing, that no matter what Garp blackmailed her with she was not going to take care of anymore of his grandsons.

Hormones:

Ace had conflicting emotions on the issue of Luffy going through puberty. He thought it was funny when Sabo came to him complaining that Luffy was taller then him. He frowned when he remembered Garp, the old bastard, laughing at his little brother when his voice cracked. And he got a weary look when he contemplated on how many times he's had to drag Luffy away from the rubble that was left after Luffy went berserk.

But this, this was a new one. Ace was embarrassed. Ace blushed. And Ace stuttered.

Luffy, the annoyance that had repeatedly followed him, the idiot that had allowed himself to get beat up for his and Sabo's friendship, and the kid that Soba and himself had adopted as a younger brother. Luffy was all those things, he was also developing hormones. And according to Garp who had learned of his, Sabo's, and Luffy's pledge to be brothers it was his job to give Luffy the 'Talk'.

Now normally explaining these kinds of things to Luffy wouldn't be that bad, it wouldn't be this awkward, but it was bad and it was awkward because every time Ace explained something to Luffy the bouncy teen wanted to try it out. And then Ace had to explain that 'No Luffy we can't do that' and 'Men can't get pregnant Luffy'. But Luffy was stubborn and kept asking questions, questions that made everything so much worse then it should be. Finally Luffy wore Ace down though.

And so Sabo walked in on Ace making out with Luffy (Sabo had quickly fled the room after giving a rather girlish shriek) which caused Ace to swiftly extract himself from Luffy's person and rush out of the room with a blush on his face.