People have up until puberty to acquire their native language with a flawless accent, which they never have to update or reacquaint themselves with. Same goes for much of the slang you pick up in adolescence, although your lexicon is more malleable.
It seems like there's more that persists than sound rules and dictionary entries, though, including the personality clues we get from the way people talk.
I've been watching or listening to a lot of the commentaries and retrospectives on the DVDs for Heathers, A Nightmare on Elm Street, Fast Times at Ridgemont High, not to mention many more over the past year or so. It's really striking how similar the Gen X people sound in their 30s or early 40s to when they starred in the movies as teenagers or early 20-somethings.
There are some changes just from being farther along in the lifespan, like not being so high-strung, as well as those reflecting the lower-key zeitgeist of the past 20 years. And some of them sound totally contemporary, like Winona Ryder who uses heavy vocal fry throughout a 2001 look back at Heathers. She sounds just like a typical bored and unexcitable Millennial chick. Jon Stewart is like that too -- the dorky leading the dorky.
Still, most of them talk a lot like they did when they were young. At first it sounds like they're stuck in adolescence -- still talking like they did as teenagers -- until you realize that teenagers don't talk that way anymore. It's more of a generational marker than an age marker. It's hard to say exactly what speech qualities I'm picking up on; it's more of a gestalt thing. But they're more expressive, more willing to open up and be real without fear of What Others Will Think. The women I especially notice using a much greater range of pitches, rather than the flat-toned speech of today's joyless young people. And they still giggle and laugh a lot! Not fake laughs either...
I also notice this around my department and any classes I share with undergrads. The Millennials are just about all of the avoidant attachment style, whether mousy or dismissive. Such a drag to talk to, often like pulling teeth, now that the mid-2000s euphoria is long gone. (They weren't so bad back then.) I've met a couple born in '85 or '86 who talk how I consider a normal person would. But I have the easiest time talking to people just around my age and back through Boomers, even the ones born in the late '40s, i.e. those who might have gone to Woodstock, unlike my parents who were born in the mid-'50s.
Probably the Millennials will be talking their way into their senior years, judging from the Silent Generation people in the commentaries, etc. For example, Ronee Blakley seemed just as reserved and almost aloof as she did as Nancy's mother in A Nightmare on Elm Street. By contrast Ray Walston, or Mr. Hand from Fast Times, sounded a lot more free-wheeling and rambunctious, even when he was interviewed as an elderly man. I went to check his birth year, and sure enough he was a Greatest Gen member (b. 1914), the early 20th-C. incarnation of what would be Gen X in the latter half (remembering that similar points in the zeitgeist cycle are separated by about 60 years).
Such a bizarre world where the middle-aged have an easier time than the youngsters at just letting it all hang out and shooting the shit with each other. It's yet another way in which we're back to the mid-century. One of the great things about It's a Wonderful Life, as far as understanding the past goes, is that it shows the past of the past, which was the Jazz Age for people of Jimmy Stewart's generation. If his character had met his wife during a high school dance of the 1950s instead of the Roaring Twenties, they wouldn't have grown as close and trusting as they did. People who go through more tumultuous times are simply less likely to take each other for granted.