I think you misread the stats by a bit. From the article:
"Together, these 3.6 million workers with wages at or below the minimum made up 4.9 percent of all hourly-paid workers."
"Although workers under age 25 represented only about one-fifth of hourly-paid workers, they made up about half of those paid the Federal minimum wage or less."
"Among employed teenagers paid by the hour, nearly 19 percent earned the minimum wage or less, compared with about 3 percent of workers age 25 and over."
"Together, these 3.6 million workers with wages at or below the minimum made up 4.9 percent of all hourly-paid workers."
"Although workers under age 25 represented only about one-fifth of hourly-paid workers, they made up about half of those paid the Federal minimum wage or less."
"Among employed teenagers paid by the hour, nearly 19 percent earned the minimum wage or less, compared with about 3 percent of workers age 25 and over."