I can explain this...
Tezuka first wrote a story called The Phoenix: Dawn shortly after he finished writing Jungle Emperor Leo... this is the Manga Shonen version.
It's an unfinished story because the magazine Manga Shonen was cancelled at a mid-point during the serialization. As far as I know, it's never been reprinted because it ended with no warning.
http://en-f.tezuka.co.jp/manga/sakuhin/ ... 35_01.html
Then he wrote three "Phoenix" stories called - Egypt, Greece, Rome.
These are stories which feature the Phoenix, but are geared to a much younger audience. They're not really part of the "true" Phoenix series. They're basically "typical" (a relative term with Tezuka) adventures stories that were printed in Shojo Club <Girl's Comic> (and a volume I've been looking for quite diligently in Korea!) - for these ones think "Phoenix Lite"
http://en-f.tezuka.co.jp/manga/sakuhin/ ... 39_01.html
After that, Tezuka fleshed out the Phoenix concept and published the series that Viz has been releasing in North America in the magazine called "COM". This is the phoenix series we know and love.
As cmoon said though, Phoenix is much more similar to Buddha than to say, Astro Boy. It's complex and has a lot of themes that may be over some younger reader's heads. That's not to say that you shouldn't read it if you want to, but be warned that you may not like it on your first pass through.
Reminds me of the time when I was 12 and just finished reading Lord of the Rings for the first time, I bought the Silmarillion at the bookstore and an "old" guy (must've been 30 at least!
) told me that it might be over my head. I was shocked and apalled that he'd say that... until I tried to read it. I finally tackled it last Christmas... and I'm still not sure I got it all...