What would you say to a teenager joining a startup?

Dude, what is the point of your question? Are you indicating that this is a foolish thing to do as the teenager will burn out before her 18th birthday, or are you saying what a fantastic opportunity it is for a extremely smart teenager with world at her feet?

Point is, everyone is different and without knowing who the teenager is, this question is, well kinda, stupid.

I mean what sort of adivce would you give him ?

I'd tell them to quit humble bragging on HN.

I'd say, "Welcome aboard, young man!"

Btw, I'm still a teenager, and that's what I wish startups would say to me when I apply to them :p

P.S. I build things with HTML, CSS, Javascript and Rails, so if anyone's interested please feel free to shoot me an email :D

Bravo