Teenage Research Unlimited, a market research firm in Northbrook, Ill., projects that the 31.6 million Americans from ages 12-19 will spend $108 billion of their own money in 2000, along with $47 billion of their family's funds. They also account for a disproportionately large share of consumer spending. Households with one or more teen-agers spend $10,000 more per year than those without any. And with parents working more than ever before, teens have assumed greater influence in household decision-making. What is the central idea of this passage?