There's a golden period between the time when your parents are making financial decisions for you and when you make your own financial decisions where your habits are in flux.

If an advertiser can reach you during that time and convince you to change behaviors, they've locked in a lifetime pattern. Pretty much everyone, for all but the few things they care about, once they form a habit, will continue on that habit until some event is big enough to jolt them out of it. Once you've decided what kind of cereal you eat and shoes you wear and car you buy, it's exceedingly hard for advertisers to reach you and change your mind.

That's why 18-35 is such an important demo for advertisers.