More than two-thirds of Americans have credit cards, and of those, about 27 percent say they have at least four cards, a recent study found. What's more disturbing is that the majority of these credit card holders have habits that result in interest charges or fees. According to the FINRA Investor Education Foundation study, one-third of credit card holders had two or more bad habits, and 17 percent engaged in three or more. The very young and the very old appear to be the most responsible. Three-quarters of Americans over 60 said they always pay their credit cards in full, followed by 51 percent of those 18-29 years old. Boomers were more likely than any other age group to be charged a fee for making a late payment (29 percent).