Information is fed into their minds in a constant stream. And by the time they can walk, they are adults.
13 seconds sound clip from the Santa Claus Conquers the Martians (1964) movie soundboard.
You can hear this line at 00:13:43 in the Blu-ray version of the movie.
Quote context
[...]
- What has this to do with our children, Ancient One?
- We have no children on Mars.
- They have children's bodies, but with adult minds. They do not have a childhood.
- I've seen this coming for centuries.
- They've never played. They've never learned to have fun. And now, now they are rebelling.
- We need a Santa Claus on Mars.
- Where will we get a Santa Claus?
- There's only one Santa Claus, and he's on Earth.
[...]