At any given moment, and in any given episode, the ages and birthdays of the six main characters can change. At one point, Monica is older than Joey, while later on Joey maintains that all three guys are the same age. Both cant be true though, as Monica is Ross younger sister, not older. Speaking of Ross though, the minor nervous breakdown that he suffers in the fifth season may have affected him more than we realised. At the end of season one, Monica says that she is 26: so if Ross is a year older, that makes him 27 in season one, and 29 in season three, something he confirms. He then confirms that hes 29 again in season four, and again in season five. At one point he tells Joey he was born in December, and at another he tells him his birthday is in October. Meanwhile, Phoebe actually has part of the plot of an episode (The One Where They All Turn Thirty) in season seven devoted to the fact that she thinks shes 30 when shes actually 31. None of them have any real clue how old they are: or if they do, theyre lying. For all we know, they could be ageless immortal creatures. Highlander was set in New York, right?