Along with its predecessor "Living Single," helped to reinvent the twenty-something sitcom wheel, ushering in an era of shows defined by a common raison d'être: hanging out.

For the titular friends, their decade-long hang began roughly in their mid-twenties, when career paths were starting to gel and a coffee shop made for a more pleasant watering hole than a bar. Rachel Green's (Jennifer Aniston) arc began when she entered Central Perk in a rain-soaked wedding gown. At the time of the pilot, which first aired in September 1994, Aniston was 25 years old.

Indeed, it's hard to clock any of the sextet's ages on "Friends" since their birthdays tend not to be set in stone. In some episodes, Rachel's birthday is said to be on May 5. Other times, it's in February (Aniston herself was born on February 11, 1969), making her age in the pilot somewhat unclear.

Aniston was among one of the youngest cast members at the time of the pilot. The youngest was Matthew Perry, who was 25, though his character was older than Rachel. Based on their high school friendship, Monica was supposed to be the same age as Rachel, but Courtney Cox was 30 years old — three years older than her "big" brother, played by David Schwimmer.

Thirty was a stressful milestone for all six friends

By the time ended in 2004, the core cast members were in their mid-thirties to early forties, but the gang had been contending with aging early in the series. In Season 4's "The One Where They're Going to Party," the guys come to terms with the fact that they're not as wild and crazy as they were in their early twenties. "I'm 29, dammit," Chandler asserts, "and I want to sit in a comfortable chair and watch television and go to sleep at a reasonable hour!"

No episode grapples with aging more than Season 7's "They One Where They All Turn Thirty." The episode focuses on Rachel's big birthday, confirming that she is, in fact, the youngest member of the group, and she's not taking the end of her twenties well. When she gets a joke card referring to her as "grandma," she bursts into tears. Later, she does some quick math, calculating the career and relationship goals she'll need to accomplish to have children by age 35. It's a wake-up call for Rachel, who promptly breaks up with her 24-year-old boyfriend, Tag (Eddie Cahill).

The episode shows all six friends turning 30 in flashback, and no one handles it particularly gracefully: Monica gets three sheets to the wind, Ross buys a sports car in a proto-mid-life crisis, and Phoebe finds out she's actually 31. For all their anxiety, however, the core characters are all fairly successful by the time they hit 30. Even Rachel, the youngest, has a cushy job at Ralph Lauren.