A new study revealed that Millennials are the worst tippers. The study, by CreditCards.com, found that 10 percent of Americans ages 18 to 37 leave no tip at all when dining out and 30 percent of that age group leaves less than 15 percent. Just 35 percent of Millennials leave a tip of 20 percent or more. So who are the best tippers? That would be seniors. According to the study, 55 percent of those 65 and up routinely leave tips of 20 percent or more.