The ages of the Friends change depending on who's talking to who, and what joke punchline the writers need to reinforce at any given time. That's to say, no character in Friends has a consistent age. In "The One Where They're Going To The Party," for example, Joey states that all three of the boys are the same age - 29. In "The One With The Birth," though, Monica says she's 26 and Joey is 25. But isn't Ross Monica's older brother?! WHY DON'T YOU KNOW HOW OLD YOU ARE, GUYS?
Speaking of ages, there must be something in that New York air that causes the gang to make up their ages on a whim whenever they feel like it. Ross, however, needs to be taken in and studied by scientists, given that he doesn't age for 3 entire seasons of show - he mentions that he's 29-years-old in seasons 3, 4 and 5, despite the fact that we've seen three individual Christmas and Thanksgiving episodes over this period, proving that time really has passed and that our palaeontologist friend isn't getting any older. That's the power of Unagi, I guess.
Over the course of the series, the writers on Friends never fully explored the fact that Ross is either an invincible alien creature from a far-off realm, or that he's just totally and utterly insane and can't keep track of his age or - uh-huh - his birth month. I mean, something is up with this guy. On two separate occasions, Ross mentions his birthday month to Joey in conversation. First, he claims it's in December. Years later, he says he was born on October 18th. And so the enigma that is Ross Geller grows ever more mysterious (and terrifying).