The risk of developing symptomatic infection itself also increased with age, about 4% per year for people ages 30-60, the study said. The authors estimated that people 60 and older were twice as likely to develop symptoms as people ages 30-59 and that people under 30 have about one-sixth the chance of developing symptoms from the infection. That suggests, as has other research, that many young people may be unknowingly infected and able to spread the virus to others.