Through the ages?

[I-Am-Atom's-Sister]

And I'm sure I read somewhere Astro was actually 7 years old in the past animes

Doctor Elfin, or whatever: "Atom may be a superrobot, but as of now, he is also a lad of 7 years old" something something school. In the Dago episode. Seen it about 18 thousand times XD

[Gotta Love ASTRO BOY]

They changed it n the movie cuz how could they kill a 9 year old

[F-Man]

And I'm sure I read somewhere Astro was actually 7 years old in the past animes

Doctor Elfin, or whatever: "Atom may be a superrobot, but as of now, he is also a lad of 7 years old" something something school. In the Dago episode. Seen it about 18 thousand times XD

Nah, he says 9 years old.

I also think the 13 thing was sort of forced just before the scene of the accident, so parents wouldn't freak out as much (if they actually would have or not is a different matter). However I would agree with the director that 13 is the age you start knowing who you are and it sort of works with this movie.

[Anime Girl]

In the movie Toby was 13, but I think the reason he's in High School is because, as you can see by how fast he took that pop quiz, he's genius and I think he skiped a grade. That would make sense right? In the manga his age would always seem to range from 3rd grade to 6th grade, which was really confusing, especially considering how most of the manga episodes are out of order. In the 80's series it said he was 9 or 8 years old.

[F-Man]

I believe in the manga he was growing emotionally at the same speed as any person, only not in size, obviously. We're told the first Astro Boy story after Captain Atom happens approximately 10 years after his birth, so by that point he is actually the same age as his classmates. There is also a story where Astro says he's celebrating his 18th Christmas.

[iceytina]

Tezuka has a little bit of information about his cuter, shorter Astro Boy versus the times he drew Astro older as well. He said in a sort of lamenting way, "he was supposed to be cool," but Astro's popularity dropped when he was older (taller), so Tezuka made him little and cute and sure enough, people loved him more again. Is there something appealing to following the adventures of a little boy more than an older boy?

It's a bit odd and like others said, based on the cultural tastes of the time: back then, people liked a cuter, younger Astro; nowadays, people like a bit of an older Astro? I guess the harder-core fans are the only ones who really care.

In the 2009 movie, he actually sounds younger in the Japanese version. He looks younger than 13 to me at least, but it's the voice that "matures him". If his voice had been a bit younger, he would have easily passed for being a few years younger. That's my opinion at least. I mean, I still think it's odd for Tenma to pick up a 13 year-old ...

[Dr. Jerk]

I have to admit, it kinda does bug me when he looks more like a teenager than a child.

[AprilSeven]

Look at Dr. Tezuka's expression in the final panel that Iceytina posted. Does he look happy?? I think not! My best guess (and, yes, my personal preference) was that Tezuka wanted Astro to continue to mature.

From his standpoint, as the writer/creator -- more story lines open up if Astro passes through different stages of human life. I'll say at least 50% of the fanfic I see involves Astro in a romance. That AIN'T gonna happen with a 9 year old boy!

And because Tezuka moved Astro's age around, it's kind of become "okay" to shuffle his character between 7-13. Much younger, and he'd be a toddler. Much older and he becomes a man.

Don't you DARE call me Peter Pan!