[furyofantares]

On your sixth birthday we put a big 5 on your cake and call you a 5 year old all year.

Can't say I've ever had to refer to someone's first year or first decade of their life, but sure I'd do that if it came up. Meanwhile, 0-indexed age comes up all the time.

[subroutine]

The number we put on the cake represents the number of "years old" (i.e. the number of birthday anniversaries) not the number of birth days someone had (obviously). Zero year-olds are 0, one year-olds are 1, ...

[drdec]

> On your sixth birthday we put a big 5 on your cake and call you a 5 year old all year.

If you are going to be that pedantic, I would point out that one only has one birthday.

(Well, unless one's mother is extremely unlucky.)

[naniwaduni]

"Birthday" does not mean the same thing as "date of birth".

[Detrytus]

“Birthday” is just short for “birth day anniversary”, I guess…

[copperx]

yes, birthday and birth day are different things. Just like everyday and every day have different meanings, and it isn't confusing (to most people).

[furyofantares]

Well regardless the number we constantly use is 0-indexed.

[layer8]

birthday != date of birth

“Birthday” really means “anniversary of the date of birth”.

[samatman]

Spanish has us beat on this one: cumpleaños, "completed year" basically.