While it has never been exactly clear how old the Friends are, we can make educated guesses. For example: we do get clues in the episode, The One Where They All Turn Thirty, it is reveals that Rachel Green is the youngest of the group, as we see all of the other Friends' have already had their thirtieth birthday.
Ross Geller is the oldest of the group, being the only person born in 1967. Joey Tribbiani Jr. is the second oldest, January 1968, albeit being the most immature. Phoebe Buffay is born in February 1968, though she grew up thinking she was born in 1969 until Ursula revealed the truth. Chandler Bing is also born in 1968, but two months younger than Phoebe being born in April.
Monica Geller-Bing and Rachel are in the same year at school, though Monica is a whole year older, being born in April of 1969 an a Rachel being born in May, 1970. All in all, the six friends are in and around the same age, the oldest being three years elder than the youngest.