In the first episode, it's the year 2000 when Dr. Tenma/Boynton starts building Astroboy. One year passes as Dr. Tenma builds Astroboy, and several years pass as Astroboy is supposed to be growing up. However, later episodes in the series say it's still the year 2000.
In several episodes, you can hear a character speaking, however when said speaker is seen, their mouth is clearly not moving.