We don't really know. .. I guess she must have been older than Veronica, Duncan and Logan who all seem to be the same age.

I'm pretty sure I read somewhere (it might have even been linked from here) that Rob Thomas had said that Lilly was a year older than the rest of the gang.

But that confused me the first time through, as well. I originally thought Veronica and Lilly were the same age, and Duncan and Logan were a year older, though that was mostly colored by my own highschool experiences rather than anything in the show.

I feel like Lilly doesn't SEEM like she is older than Veronica-- I mean, when I was in high school I can't think of anyone who was "best friends" with someone not in their grade, unless they were cousins or something. Wouldn't Lilly have been best friends with Shelly Pomeroy (assuming Shelly is a senior) or something, instead of Veronica?

eta: I assumed what Kalshane assumed-- or perhaps that Duncan was older than all of them

The exact age of our fearsome foursome is a point of much debates among the Marsheads (Mars-holics? Marshettes?) This is from the timeline at marsinvestigations.net (of which P-C is one of the editors):

January to October 1987

Lilly is born. 1987-2003 is engraved on the memorial fountain. Since Lilly drove on the day of her murder, it means she had already turned 16, therefore she was born before October. (1.04 "The Wrath of Con")

February 13, 1987

Duncan is born. February 13, 1987 is listed as Duncan's birthdate on the Missing Person Link website. (Rick Pickett revealed in his interview that he made up Duncan's birth date. He was not aware that the writers had an official list with the birthdays of the characters, therefore this date should not be accepted as canon.) (1.20 "M.A.D.")

April 1987 to May 1988

Logan is born. (Logan tells Aaron he intends to leave once he turns 18 in 1.21 "A Trip to the Dentist".)

May 1987 to September 1987

Veronica is born. Since Lilly was alive at the time of Veronica's sweet 16 birthday party, it means Veronica was born before October 3, 1987. (1.01 "Pilot")

The timeline (contains spoilers up to the aired episode.)

I thought Lilly and Duncan were fraternal twins for the longest time; I do remember RT saying something about Lilly being older, but can't recall from where. Possibly one of his TV Guide interviews.

It was in his TWoP interview. Also, there's a point last season where Veronica notes that Lilly would have been looking at colleges if she were still alive, which means she was at least a year ahead of her. You can also take Lilly's comment in "Lord of the Bling" about younger brothers to mean that Duncan is in fact her younger brother.

Thanks Vonnie! Also, I am enjoying your crackvan Veronica Mars recs so far!

I think they should just make Lilly and Duncan fraternal twins-- or would that be too Brenda and Brendon from 90210?

You're welcome, Sophia. That story of Yahtzee's is amazing, isn't it? I was surprised that it wasn't recc'd already.

Because of all the storylines about questionable parentage in addition to Lilly's and Duncan's wacky birthdates that seemingly defied human reproductive biology, there was also some speculation that Lilly might not have been Jake and Celeste's daughter. Which has been put to rest now, more or less. (During the last month of S1, before the reveal about Veronica's rape, her paternity, and Lilly's murder, the fandom went completely insane with all kinds of speculations, both plausible and harebrained. Ahhh, fun times, fun times.)

They could be twins and Lilly still "older" than Duncan (it would actually make the "little brother" thing even funnier) Although surely there would have been comment. I could even buy that he was a year behind her in school, if there was some issue with diagnosing his medical issues.

Still the best explanation I can think of.

It also explains why she had such close ties to V's class (although I'm apparently the aberration who had a best frind in another class, but I expect that theatre is an exception-prone circumstance)

I think the actress who played Lilly is closer in age to her character, than the other three are to their characters. That said, I've also read where Rob Thomas said Lilly was a year older than Duncan.

I always assumed after the little brother comment and the "looking at colleges" line meant that Lilly was older. of course, i'm also someone who was best friends with someone who was two years younger than me. *shrug*