> Being very much a kid (at heart at least), I'll wager the question: what's the case against declaring variables as you need them?

There's not really one, it's just a joke about how some languages used to work.

> Also, at the top of which scope are you supposed to declare variables, function scope, class scope, ... somewhere else?

In C, you used to have to declare all local variables at the top of the function rather than throughout the body.