How old are the characters throught

Chuck Bass—May 19, 1991 (17)
Blair Waldorf—November 17, 1990 (18)
Serena furgone, van der Woodsen—July 14 (17)
Nathaniel Archibald- 17
Dan Humphrey- 18 ( they say it at 18x02 )
Vanessa Abrams - 17
Jenny Humphrey - 15 (she will turn 16 in 20x02)

Well, in The Age Of Dissonance Dan says he's 18.. :)