I'll start off with an exchange between Leonardo and Splinter from the recent Teenage mutant ninja Turtles comic book series "Desperate Measures" in which the former vents his frustration about having to deal with a team consisting of teenagers.
Leonardo: It's just - There's so many bad things going on and ALL they want to do argue with each other and goof around. I TRY to get them to focus, to take all of this seriously and they just BLOW ME OFF at every turn. Raph wants to HUNKER DOWN, Mikey wants to be a SUPERHERO and Donnie's got his HEAD BURIED in his gadgets 24/7. They're driving me CRAZY!
As he finishes his rant, Leonardo notices that Splinter is grinning.
Leonardo: It... It's NOT FUNNY Father!
Splinter: I am not laughing my son.
Leonardo: But you're GRINNING!
Splinter: Yes, yes, I AM doing that. Forgive me Leonardo, my amusement is not to be insensitive. I am just happy to know that my Teenaged boys are STILL teenaged boys even when they are away from me.
I can't speak for others but I think having the "Teenage" aspect of Teenage Mutant Ninja Turtles being the main focus of the 2012 series was a master stroke because as others pointed out, in other versions of the show (The 2003 series, awesome as it was, being the most guilty of this) the turtles weren't behaving like teens.
The skating scene and the sneaking out of the lair when Splinter isn't watching in "I think his name is Baxter Stockman" is one of the best examples of the Turtles behaving like common hoodlum teenage boys.
"So you wouldn't object to a little RANDORI?"
The Turtles got their human side from either Splinter or the clerk of the pet store where they were bought, there might have been several humans who touched them before their mutation, which also explains why they all developed into such different personalities.
One thing about boys in their teens is that those are the days when they start to discover the opposite gender, so with an incarnation of the TMNT where they are, well...TEENS, it made good sense to make the main female characters (April, Karai, Renet) teens too.
I must say though that of all the shippings here, Karai and Leonardo was the most natural, the most authentic, that feeling of "I shouldn't be as infatuated with her because I know she's bad news but I AM!" is something we all experienced during our own teen years. As was their bantering, for that matter. If Leonardo is meant to be the oldest brother of the Turtles, Karai was written in as somebody who clearly is older than April, she still has that teenage rebellion going on but with a sense of "I've been around, I know what I'm talking about." kind of deal that teens who are on the edge of seventeen are.
Now take the scene in which Leo tells April about Karai, at first she's "Oh I'm so glad for you." and when he admits that Karai is in the Foot Clan, she slaps him in the face and declares him crazy. Which again is a very authentic teenage thing to do, teen girls can be ruthless about other teen girls after all.
"SHE'S IN THE FOOT CLAN!"
How would that episode have been if April had been in her twenties?
So I have no issue with a Teenage April, okay she wasn't the big sister type she was in previous versions, she also wasn't the LITTLE sister her current counterpart is now.
