Identifiers

If we assume that there are 8.5 billion people in the world, then what could make a decent unique identifier for each one?

If we used binary (1 or 0), it would need to be 33 characters long. Probably not the easiest to read though.
A simple number (0-9) - this would be 10 digits long.
If we made it hexadecimal (a-f0-9), you'd only save 1 character - 9 characters would be needed.
Purely lowercase letters (a-z)? 7 doesn't quite do it - you'd need 8.
Alphanumeric (the standard English 26 character alphabet) (a-z0-9)? 7 would be required.
Mixed upper and lower case (a-zA-Z)? 6 would more than enough.
If we add numbers to the mixed case letters (a-zA-Z0-9), we'd still need 6 characters.

You'd need a combination of 97 characters in order to get it down to 5 characters.

And if we extend this to include all the people who have ever lived (guessed at about 120 billion), we'd still only need 7 characters of a-zA-Z.