We’re becoming old. Tragically, inevitably, it’s happening.

Late-night yackbox and “Tonight Show” host Jay Leno officially announced on Monday, Sept. 26 that he would end of his reign of blandness in early 2009, stepping aside and allowing Conan O’Brien to assume hosting duties. O’Brien, the man whom NBC has tucked away in the post-Leno “Late Night” timeslot since 1993, will replace Leno’s stale jokes with a quirky, original and often downright bizarre humor. He’ll just have to wait five years, a time period that equates Leno to roughly six good jokes.

In March, I wrote a column decrying NBC’s decision to sign Leno through 2009 because I felt it ignored the legion of college-aged fans Conan has garnered in the 11 years since the network signed a next-to-unknown writer from “The Simpsons” and “Saturday Night Live.” He is catnip to us, for the most part, and he knows how to mix pop culture references with his own comedy innovations – a phenomenon perhaps best represented, of course, by a chef’s hat-wearing cactus that plays “We Didn’t Start the Fire” on a flute. Leno, conversely, delivers jokes like he’s tossing half-ton bricks made out of shit. But I digress… Rather than wait around for Leno to shove off, O’Brien might have fled to a kinder network – or left the late show business altogether.

So one would think I would be elated to have O’Brien in line to move into the earlier timeslot. I am. However, that looming date of 2009 has reminded me about the inevitable passing of time – and the underlying wisdom NBC exhibited in keeping Leno around.

By the time Conan O’Brien becomes the fifth host of NBC’s “Tonight Show,” most of the people reading this column now will be all grown up. Most of us, I hope, will be done with our undergraduate work. We’ll have jobs, spouses and maybe even children. Some, conversely, will have no spouses, no children and a lot of cats – but they’ll still be the appropriate age. We’ll still like Conan, but we’ll have to get up for work every morning. We’ll be old, and we’ll probably be home in time to see his show every weekday night.

The age set that keeps Leno popular will also be more finely aged – maybe to the point at which 11:30 p.m. is past their bedtime. And those fogies who can keep their eyes peeled through O’Brien’s opening monologue would likely be put off by his antics, what with his hissing and mugging and playing of “Walker, Texas Ranger” clips.

In retrospect, the extension of Leno’s contract through 2009 makes a lot of sense, because that’s how long a shift in the adult demographic will take. That’s doesn’t mean the transition into post-college student, post-young people, post-cool age group is any less unnerving for me.

To put time in perspective, 2009 will mark the 10th anniversary of movies like “The Matrix” and shows like “The Sopranos” and “Family Guy.” By this year, Bill Clinton’s impeachment hearings will be a 10-year-old footnote in American history. This year is also the scheduled completion date for the Freedom Tower, the structure replacing the World Trade Center, which fell in terrorist attacks that will be eight years old. And the weirdness that once occupied the 1 a.m. timeslot, in the form of off-the-wall fringe comedy invented by a hyperactive Irish giant, will suddenly become the mainstream.

Hurray for Conan O’Brien and a “see you in Hell” for Jay Leno. It’s good to know that mediocre comedy must one day bow out to something that might actually make you laugh. But maybe – just for a second – the idea of getting older is scarier than even another season of Jay Leno.

Daily Nexus columnist Drew Mackie both hisses and mugs when he plays clips of “The Tonight Show.”