In the first season, when Chandler goes to Joey's tailor to get fitted for a suit, Joey tells about his first time at the tailor. He says that he was 16 in 1990. But later in the episode, Joey claims to be 27. Is it 2001 yet? No, this episode is from 1995!!
Yeah but remember it's Joey, and he's not the brightest bulb on the Christmas tree, if you get what I mean. Maybe this mathematical error was in keeping with Joey's character?
Joey doesn't actually say he was 16 in 1990, he just asked, "when was 1990?" I know he had been saying he was 15 or 16, but his 1990 reference could be to something else to remind him of how old he was.
Joey isn't exactly a wizz in maths is he. You know the episode where they try to convince Phoebe's brother Frank not to marry Alice, he says "You're 18, she's 44... when you're 36, she's going to be 88..." she would be 62 actually, Joey.
Errors like this are rife throughout the series. If you want a real challenge, and if you've got the patience, try figuring out Ross's birthday based on information that appears throughout the entire ten seasons, and working on the assumption that the show more or less coincides with "real time" - i.e that the original U.S. airdate is more or less when the show is set (for example, a show that deals with Thanksgiving will be aired more or less to coincide with Thanksgiving in real life). I gave up at the point where the show had established three seperate birthdays for him.
the idea is that Joey thinks he was either 15 or 16 and couldn't figure out which one. He then says "ok, when was 1990?" as a joke for the audience to realize that he was much older than 15 or 16 when he got tailored because it was 1990. its all for humor!