[He snorts at the question and pauses, deciding on how he wants to answer.]

Technically speaking, you're the youngest. [You know, if non-years get to count. Which they don't.]

Although from a legal standpoint you're the oldest. Especially since me and Hazel haven't worked out the whole social security number thing. [He doesn't know what a social security number is, he just knows Chiron keeps mumbling about getting Nico one because he's Supposed To Have one in order to exist as a person in the mortal world.]