Phoebe was 29 when she found out that Phoebe Abbott was her biological mother on Friends.
The revelation unfolds in the season-three finale, “The One at the Beach.” Phoebe travels to Montauk to meet a woman she believes was her mother Lily’s old friend. During the visit, Phoebe Abbott admits that she is not merely a family friend: she is Phoebe’s birth mother. The discovery explains why Phoebe’s family history has always been unusually confusing.
Phoebe had believed Lily Buffay was her mother. Lily raised Phoebe and her twin sister, Ursula, but later died by suicide when the girls were teenagers. Their biological father, Frank Buffay, had also disappeared from their lives. Phoebe therefore grows up with major unanswered questions about where she came from.
The age is confirmed in the following episode, “The One with the Jellyfish,” when Phoebe angrily tells her birth mother that she had 29 years to find out the truth. Friends later plays loosely with the characters’ ages, but this specific storyline identifies Phoebe as 29 at the time of the discovery.