Once you understand the differences between 1,10, 100, then if you can't extrapolate to 1000 and into infinity then you don't really understand the difference between 10 and 100. They must not be teaching it well enough.
What I remember from my own childhood is that they made it "harder" by giving bigger and bigger problems with endlessly dull computation, like "259,384 + 513,628." This is not conceptually any more difficult than simpler place-value and carrying problems, but it requires a heck of a lot more patience and attention and executive control. Hence, older kids are ready for it, younger kids aren't. QED.
