Jennifer Aniston was 29 years of age when the TV sitcom 'friends' was first aired. Courtney Bass Cox was 28 years old while Lisa Kudrow was 31 years of age. Matt Leblanc was 27, Mathew Perry 25 and David Shwimmer was 28 years old. Visit http://en.wikipedia.org/wiki/Friends, for more information on the sitcom.
I'm not sure if you mean real age or the age of their characters so I'm going with how old they were in real life.
Chandler (Matthew Perry) - 25 years old Rachel (Jennifer Aniston) - 25 years old and turned 26 by the end of season 1 Joey (Matt LeBlanc) - 27 years old Ross (David Schwimmer) - 27 years old and turned 28 by the end of season 1 Monica (Courtney Cox) - 30 years old Phoebe (Lisa Kudrow) - 31 years old
They also had an episode where Rachel turned 30 that aired in 2001 (when she was actually 32) and it was centered around her 30th birth party and flashbacks to the other character's 30th birthday parties. Which means Rachel was the youngest of the group. Also, we're led to believe Ross and Chandler are the same age since they were college roommates, and Monica and Rachel were the same age since they were high school classmates. Given that Ross was Monica's older brother, I would guess he and Chandler were 31-32 when Rachel turned 30, and Monica was 30-31. However, Ross/David is 2-3 years older than Chandler/Matthew in real life, and Monica/Courtney is 4-5 years older than Rachel/Jennifer in real life.