I understand what you are saying, but it's a bit ironic that the only demo that seems to appeal to these big money agencies is merely 15 years in breadth. Even less, if you consider that many young adults don't really begin making decent money until they leave college at age 22, which would make it a 13 year spread -- age 22 to 35.
Advertisers are looking to develop brand loyalty as early as possible. If the 17-year-old can't drink Budweiser legally now, he will be able to in a few years, so saturate that brain with images of happy, cool people drinking Bud. A 40-year-old Coors drinker is useless; he knows what his favorite brew is and will just tune out the rival brands' ads. Same goes for cars -- the 50-year-old who's owned five Dodge Ram pickups isn't going to buy a Ford next time, but the 16-year-old with a learner's permit is a blank slate, so imprint the Ram brand and hope that's what he asks Dad to get him after he graduates from high school.