About 4 in 10 adults in the U.S. are at greater risk of developing serious illness if infected with coronavirus, due to age or underlying health conditions

From the article:

Of the more than 105 million adults at higher risk if infected with coronavirus, most – 76.3 million, or 72 percent – are age 60 or older, the analysis finds. However, the remaining 29.2 million adults in this group are ages 18-59 and are at higher risk if infected due to an underlying medical condition such as heart disease, cancer, chronic obstructive pulmonary disease (COPD) or diabetes.

Nearly 6 million people at higher risk are uninsured, including 3.9 million adults under age 60 and 1.8 million who are ages 60-64. (Virtually all adults ages 65 and older are covered by Medicare.)

(Also, there is state-level data.)