Finally began messing around with C... much is coming back to me now, and realising how little I know.

if.c

#include <stdio.h>

int main(void) { // Thanks to @barrettcolin for showing me the way.
        int age;

        printf("What is your age? ");
        scanf("%d", &age); // Accept a numeric input and push it to age (by getting the lvalue of age)
        if(age < 5) {
                printf("Wow, you're very young!\n");
        } else if(age < 12) {
                printf("Oh, older now!\n");
        } else if(age < 18) {
                printf("A teenager? Party everyday!\n");
        } else {
                printf("Sorry, you're an adult now. Parties with beer (and whiskey)!\n");
        }

        return 0; // Oops, forgot this originally. Though as it turns out there was no need to do this at all in C99
} // Also forgot the newline at the end.