• by Adam Buckman , Featured Columnist, 2 hours ago

Everyone in the cast of “It’s Always Sunny in Philadelphia” has aged 21 years since the show premiered in 2005, but their characters, along with the show, remain the same.

You would hardly know the show is 21 years old when watching the premiere episode of the show’s 18th season, coming Monday on FX and Hulu.

Maybe living in sunny Southern California, where this Philadelphia-based comedy series is produced, can add years to one’s life. The region is certainly sunnier than Philadelphia. 

When “It’s Always Sunny in Philadelphia” premiered in 2005, its four principal cast members were still in their 20s.

Like “Seinfeld,” the foursome consists of three men and one woman. The actors playing them are just about equal in age -- Charlie Day (“Charlie Kelly”), 50, Glenn Howerton (“Dennis Reynolds”), 50, Kaitlin Olson (“Dee Reynolds”), 50, and Rob McElhenney (“Ronald 'Mac' McDonald), 49. The show’s fifth marquee star is Danny DeVito (“Frank Reynolds”), now 80.

One of the hallmarks of this show is that all through these many years, the characters have undergone none of the changes in behaviors and attitudes that most of us experience as we age.

They are like the cartoon characters in shows such as “The Simpsons” and “Family Guy.” Since those shows are animated, none of the characters need to age at all.

In much the same way, the four characters in “It’s Always Sunny in Philadelphia” are also unchanged. 

They are still as immature and prone to silly arguments and inappropriate behavior as they were on the day the show premiered 21 years ago. 

As the new season begins, it is Frank’s wedding day and he is petrified. And he is not helped in any way by Charlie, Mac and Dennis, who are trying to soothe his anxieties. True to form, their suggestions and actions plunge the situation into anarchy.

Meanwhile, Dee is conspiring with three friends to derail the marriage by persuading Frank’s fiancée Samantha (Carol Kane) that Frank isn’t worth it.

The episode includes the appearance of a deranged ventriloquist dummy, Mac’s chain-smoking mom and the cremated ashes of Charlie’s mom -- and much, much more.

At 18 seasons, “It’s Always Sunny in Philadelphia” is the longest-running live-action comedy series in the history of television. 

“It’s Always Sunny In Philadelphia” starts its 18th season on Monday, August 17, at 9 p.m. Eastern on FX, and streaming on Hulu.