In the books, Chuck is older than Blair, because his birthday is January 19, 1991 and her's is November 10, 1991. However in the show, Blair's birthday is November 17, 1990 and Chuck's birthday is January 19, 1991. I think they wanted to keep her birthday in November, but couldn't make it in 1991 without her being in the grade below everyone, meaning that if they kept her birthday the same, Chuck, Nate and Serena would be in college while she was still at Constance. Also, with the mishap of Chuck's passport & Dan Humphrey's essay, Chuck was born on January 19th, NOT in May! I think I'd trust a passport over Dan Humphrey's journalism and investigative skills!
I'm certain he's younger than Blair. Think about it, he was born on 19 may 1991, and Blair in 1990 ! That's sure, because at the beginning of season 2, it's Blair's birthday in november, she is 18 and Chuck is ... still 17.
In episode 14 oder 15 of the season 2, Lily tells Chuck he will have bass compagnies at the age of 18, so he's still seventeen !