There's always a lot of talk about how 40 is the new 30, but it usually means that people are doing things at the age of 40 that previous generations did at the age of 30. I think, though, that 40 is also the new 30 in entertainment, specifically in time travel stories and in flashbacks.
I remember noticing this for the first time when I was watching Battlestar Galactica: Razor, which has flashbacks to the Cylon War four decades previous. At first that struck me as a gigantic span of time - how old was Bill Adama anyway? But then I did the math, and figured if he was flying against the Cylons in his 20s, and running away from them in his 60s, then it made sense.
What'd you say about running away, punk?
Contrast this with Back to the Future - if you wanted a time to contrast with 1985, then 1955 was a pretty good bet. For one thing it felt properly remote, but not so much so that Marty's parents would be too young - or, in the 1985 portions of the movie, too old. And frankly, 30 years was pretty much all the parents were rated for - back in the 80s a person in their 60s clearly had one foot in the grave. Whereas now, someone in their 60s can reasonably look forward to about 20 or 30 more years of rollerblading and managing their diabetes (which is what old folks get up to, if TV commercials are to be believed).
"Time to plan my retirement, then roller-blading 'til I drop!"
If you need another example, Men in Black 3 does go back in time less than 40 years (back to 1969, to be precise), but I think it still contributes to my thesis. Going back exactly 30 years would have taken Will Smith's character back to 1972, and the 1970s have a different feel to them, in the movies at least, than the 1960s.
I'm sure there are further examples and counter-examples, enough so that we could sit here and argue about it until Nike releases those self-tying shoes and our bosses can fire us by fax. But I think it's interesting how entertainment is adjusting to demographic shifts in longevity.