Ever since I turned 30 a couple of years back, I keep running into little milestones – events and objects which remind me that, while I’m still young, I’m not YOUNG young anymore.

On a related note: John Cusack turned 40 last week.

You’re probably wondering why this should matter. Cusack, after all, never has been one of Hollywood’s $20 million actors. He’s not quite an A-list star, but he does hover somewhere above the throng of familiar-faced character actors.

Ever since I was an adolescent, though, John Cusack has been a steady, reliable presence in the movies – often in the movies my generation used to define itself. He’s probably the defining Generation X actor, somebody whose passages from teen angst into young adulthood into looming middle age have accompanied, onscreen, our own journeys.

Think about it. John Cusack has a defining film, or two, for every era of our lives:

¢ High school: He served his requisite John Hughes time in the 1980s, appearing in “Sixteen Candles.” But it was his star turn in “Better Off Dead” – “I want my two dollars!” – that made Cusack a hero in my high school.

¢ Early, post-high school adulthood: “Say Anything.” That’s the movie where he stood outside his girlfriend’s house, boombox over his head, playing Peter Gabriel in some kind of postmodern serenade.

¢ Sellout stage of adulthood: “Con Air.” Just a wretched, wretched movie.

¢ Just turned 30, trying to figure out whether I want to be a kid all my life: “High Fidelity,” where he played the record store owner wrestling with issues of fidelity. And “Grosse Point Blank,” where he played the hitman coming home for his high school reunion – trying to figure out if he’s become the kind of adult he wanted to be.

Most guys I know wanted to be John Cusack’s character in “High Fidelity” – with a rich, deep, obsessive passion for music. Many women I know hoped to find their own Lloyd Dobler from “Say Anything.”

Is there any other actor of his generation who has been part of so many iconic moments, or endured on the screen for so long? Kevin Bacon is the only actor who comes to mind.

And now Cusack is 40. Which, they say, is the new 30, so he might have a couple of years of coming-of-age “Peter Pan Syndrome” films left in him.

Somewhere in the next couple of years, though, he’ll start slipping into the “dad” roles, the kind that Steve Martin has been doing in his sleep since “Father of the Bride.”

And people my age will track their own progress as they watch him grow, finally, into irrevocable adulthood.

Happy birthday, John Cusack.