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? Also, at the top of which scope are you supposed to declare variables, function scope, class scope, ... somewhere else?
Minimizing the scope of variables decreases the amount of scanning/grokking you have to do to find out how they are related. (Making them constant too also helps a lot as well... you know it won't change thereafter)
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.
This is true, but support for block-scoped (as opposed to function-scoped) variable declarations was added to C a long time ago. Every version of ANSI C supports them, and this stack exchange answer, https://softwareengineering.stackexchange.com/a/300274/19196, suggests that support was added to K&R C since at least 1978.