I first became aware of it around the time I got out of college. Without reading the article, my thought process on it was that when you're young, each year, or any amount of time, for that matter, represents a larger part of your whole life experience. For instance, when you turned 10, that previous year was 1/10 of everything you've ever experienced. And probably more, for that matter, as you probably don't remember much, from your first few years of life.
But when you turned 20, that previous year was only 1/20th, and so on.
I think another thing that might happen, is that other than a few major life events here and there, life often becomes more repetitive as you age. When you're young, things change fast, as you grow. You learn to walk, talk, go through nursery school, kindergarten, elementary school, junior high, high school, possibly college. You're also constantly learning along the way, and your body is growing, and changing fast. Going through puberty is usually a pretty big milestone. So is learning to drive. And moving out on your own, getting a job, getting married, having kids, and so on.
But then, as you get older, it just seems to get repetitive. You might move every once in awhile, get a promotion/change jobs, take vacations here and there, but it still often just falls into a pattern. Things still change, but it's still not the same as the first time you did something. For instance, you're most likely going to buy a new car every once in awhile, but even that becomes repetition. It's not the same as the first time you got a car.
At least, that's my theory on it. YMMV.