The short, cynical version is that after about 35, people are too old and wise for advertising to work on them. That’s meant tongue-in-cheek, so please give me a chance to explain, but there really is a rational reason why advertisers covet the 18-35 demographic.

Before getting into the meat of it, I should clarify that there are two main types of advertising. For convenience, I’ll call them persuasive advertising and product-awareness advertising. Product-awareness advertising is telling people about products they don’t know exist. Examples might be a new kind of jar opener, but probably more common are things like books which have just been published. This sort of advertising works on all ages, but in general on such a small subset of people in all of those ages that it is rarely worth it in general-audience advertising. Highly targeted advertising, like Amazon advertising and Facebook advertising can work fairly well for it, where there is more demographic data available than approximate ages.

When it comes to advertising which is to persuade people to try something, rather than to let people know that it exists, this will have a spectrum of efficacy. There are few people immune to truly enormous amounts of persuasion, but persuasion by advertising is expensive. There are, therefore, only two sorts of things for which it makes sense to spend a lot of money trying to persuade people to buy:

1. Extremely expensive things
2. Things people will buy an awful lot of

Thus we have the two main categories of things we see advertising for: very expensive things like cars, and things we use a lot of, like beer and toothpaste. These all have something in common, though: people have a tendency to find a brand they like and stick with it. This is perhaps less true of cars than it is of toothpaste, but only so much, and I believe that car advertising tends to be less concentrated in its demographic preferences.

Because we tend to develop brand preferences—which is, itself, a rational optimization of mental effort; once we find something good enough we can spend time on other things which bring us greater benefit—it is cheaper to persuade people to try a product who do not yet have brand preferences. The strong the brand preferences, the more expensive it will be to persuade people to try to give them up.

There is a second factor which pulls in the same direction. The younger you are, the more years you will be buying toothpaste, beer, etc. and therefore the greater the return on the investment of persuading you to trying their brand.

These two factors produce a double-whammy: it is at once more expensive to persuade an older person to try your product and the return you will get for success is smaller. It is clear, therefore, that the preference advertisers have for the 18-35 demographic is rational.

It is the case, of course, that there are season to life, and at each stage in life there are products one is considering for the first time. There’s no point in trying to sell luxury cars to people in the 18-35 demographic, almost none of them have the disposable income to buy such luxuries. There’s no point in trying to sell Depends or Ensure to the 18-35 demographic, because they don’t need them. Older demographics still are rational for advertisers to market to, it’s just that the markets tend to be more niche or otherwise smaller. You might realistically sell toothpaste to someone for fifty years or more; you probably won’t be selling adult diapers to anyone for even thirty years.

The reason why I bring all of this up is that I believe that this is a structural phenomenon, not merely a passing fad from the youth culture of the 1970s which still hasn’t gone away. Accordingly, anything which is paid for, not by the people watching it, but by advertising being shown to the people watching it, will have a strong tendency to come to focus on trying to capture the 18-35 demographic.