As for what makes Astro look more mature, in the manga itself it was partly his stature and bearing. In both the manga and this little collectors' card, it's the eyebrows.
![Surprised :o](./images/smilies/icon_e_surprised.gif)
It's too bad that no one can find more of that Mars postcard; I hadn't even known that a version like that had been made! Of course, I didn't know about the Return of Atom look either, until I found it by accident in an image search! (I can't remember my search terms for the life of me; I feel like it was when I looked up Jetter Mars in Japanese, though. Found some other cool stuff, too.)