US journal Perspectives in Sexual and Reproductive Health says half of America's 18 million STD cases belong to this age group even though they account for only one quarter of sexually active people.
Human papillomavirus (HPV), trichomoniasis and chlamydia are the three diseases that affect nine in 10 of all new cases of STD infections in the group, the journal said on Tuesday.
"It is not surprising that teens and young adults contract a disproportionate number of infections," said Sharon Camp of the Alan Guttmacher Institute which financed the study.
"Most young people are sexually active, and many are ill equipped to prevent STDs or seek testing and treatment," Camp said.
New estimates suggest the direct medical costs associated with a lifetime of treating cases of STD infection diagnosed in young people in 2000 could reach $6.5 billion.
Ninety percent of that cost, however, is due to treatment of HIV/AIDS and the small percentage of cases that result in cervical abnormalities or genital warts.
"It is not surprising that teens and young adults contract a disproportionate number of infections. Most young people are sexually active, and many are ill equipped to prevent STDs or seek testing and treatment"
Alan Guttmacher Institute
The authors of the study emphasize prevention and education to cut the future cost of STD infections.
"Investing money today in STD prevention and education could dramatically reduce the incidence of these infections, and thus future treatment costs," the authors said.
Adolescents "need realistic sex education that teaches them how to prevent STDs and unwanted pregnancies", Camp said.
"It is essential to have medically accurate information about condoms and other contraceptive methods, and guidance in how to access appropriate prevention, testing and treatment services."
A number of STD prevention programmes in American schools, financed by the public purse and by conservative organisations, try and teach sexual abstinence without mentioning condoms to prevent disease in the event of sexual relations.