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They didn't actually know their exact ages.
Donnie had come to that realization.
Sure, they knew the day they'd been mutated, but they had no idea when they had actually been born.
They had simply fallen into their roles: Leo was the oldest brother, Donnie and Raph were the middle siblings, and Mikey was the youngest.
It had always been that way.
But Donnie had never stopped to think about how much younger Mikey really was. Sure, he acted like a little kid, but that was because he'd always been treated like one. Donnie knew they had encouraged that behavior, even if unintentionally.
Still...
They were all eighteen now, right?
That's what he'd believed for years.
And he was certain that he, Leo, and Raph were.
But Mikey...
Mikey looked like he had only just gone through puberty.
Everything about him had changed, and it was impossible not to notice.
It was starting to worry his older brothers.
"So... does that mean when we were fifteen, he wasn't fifteen too?" Leo asked before taking a sip of his juice.
"I don't think so," Donnie replied, staring into his coffee as if it could somehow provide the answers.
"So how old was he?" Raph asked.
It was a good question.
From everything Donnie had pieced together, Mikey had been around twelve when the rest of them were fifteen.
Now they were eighteen...
And Mikey had only just turned fifteen.
"Why didn't anyone wake me up for breakfast?"
Mikey walked into the kitchen while the three of them watched him make himself a sandwich.
He had finally caught up in height, now standing eye to eye with Raphael. His shell had stretched out as well, and he looked noticeably leaner.
Donnie had come to think that Mikey's once-round shell had simply been the equivalent of baby fat.
His freckles had spread across almost his entire body now, dusting his shoulders and arms before climbing up the back of his head, where they formed something that resembled an arrow.
His new bandana was as long as theirs. They'd had to replace the old one after it had been torn during a fight.
The scar that started on his lip and reached nearly to his eye remained as proof.
"Is there any juice left, or do I have to make more?"
And his voice...
That high, raspy voice was finally gone, replaced by one that was deeper...
Older.
Michelangelo had grown up.
Sure, he still joked about everything and never missed an opportunity to tease someone, but now there was a calm confidence behind it.
A dry wit.
There was something undeniably charming about his new personality.
Donnie had begun to wonder if Mikey had never actually been childish in the way they'd always assumed.
Maybe he'd simply been a child acting like a child while surrounded by teenagers.
And now...
He had finally reached adolescence.
While his brothers had already become adults.
"You'll have to make more," Leo answered, holding up the empty juice bottle.
Mikey let out an annoyed sigh.
"Seriously? If I'd finished all the juice without leaving any for you guys, I'd never hear the end of it."
And the comebacks...
Mikey always had a sarcastic remark ready.
At first, the three of them had been shocked to see the youngest sibling arguing with Raphael as an equal.
Now they were slowly getting used to this new version of their little brother.
The three of them had already talked it over and agreed there was no reason to tell Mikey about the difference in their ages.
Still, as the oldest brother, Leonardo couldn't help feeling guilty whenever he lay awake at night.
Everything they'd been through...
Every mission.
Every battle.
Every life-threatening situation.
All that time, Mikey had just been a child forced to carry responsibilities no child should have.
Leo could only hope it wouldn't steal away the adolescence he was finally getting to have.
"You know what? I'm not really hungry. I'm meeting a friend, and I don't want to be late."
Mikey grabbed his things and hurried out of the kitchen.
That was another thing about Michelangelo.
His brothers had to admit he was growing into a handsome young man.
And with more and more mutants becoming part of their lives, it was no surprise that plenty of girls seemed eager to spend time with their little brother.