Officials said the unidentified teen lived in the Los Angeles-area suburb of Lancaster.
Both of the additional cases reported Friday are male and residents of Garfield County.
"This is a devastating reminder that COVID-19 affects people of all ages", said Barbara Ferrer, the director of the county Public Health Department, according to Fox News affiliate KTTV.More news: Fed govt apologises for Centrelink queues
Health officials in New York City and Los Angeles are recommending that doctors avoid testing patients, except in cases where a test result would significantly change the course of treatment. Although severe cases of COVID-19 are more common in the elderly, a majority of those infected have been younger.
Why it matters: The fatality rate for COVID-19 in the U.S.is highest among those over 85, the CDC found in a report issued last week - and no deaths among people under 19 years of age were reported in the country as of March 18.More news: N.Korea fires two short-range ballistic missiles: Yonhap
"Each loss we experience in LA County is tragic, and we are sending our heartfelt condolences to the families and loved ones who've had to endure this tragedy", Ferrer said.
"Garfield County will follow all Public Health Orders issued by the Colorado Department of Public Health and Environment (CDPHE) and all Executive Orders issued by the Governor of Colorado related to the COVID-19 virus unless otherwise excepted or ordered."
Cases in California have topped 2,500 with at least 50 deaths according to figures from Johns Hopkins University.More news: Microsoft Announces a 'Free Educational Marketplace' for Minecraft
This is a developing story and will be updated with more information.