There are multiple reasons.
One of these is adults to tend think that those times were the best times of their lives, when we view the lives we had were much simpler. Especially since much of the high school type drama we experienced we still find in places like the work place even when not dealing with teens. Which is another partial reason some people never seemed to mature beyond that, ironically they think people who read comics, play video games are the immature ones.
The youthful themselves are part of the problem, even in a day where the economy is like it is to a kid a quarter is like a dragon's treasure trove. You talk to teens you find them saying things like I have a lot of money, I mean a lot. When you find out how much it's a hundred bucks and they think they are rich. They still think making minimum wage is a lot of money. In the case you made a 15/16 year old is going to take less money than adult because they have a different concept of what a lot of money is, partially because they haven't built up the bills yet.
Some of this is cyclical when you look back through the past.
there are other reasons also, like the shift in how things are marketed to the younger generations. the fact that as we get older we tend to not get into new stuff, for example ours isn't the first generation to call the music the kids listen to as nothing but noise, this was said of the Beatles, and many other bands and they were also too sexual.