The nation’s youngest adults remain the least likely to be vaccinated against the coronavirus — and their weekly rates of vaccination are declining, according to federal research released Monday. The CDC analyzed adult vaccination rates by age through May 22, finding 80 percent of adults older than 65 had been immunized compared with just 38.3 percent of 18- to 29-year-olds. The percentage of people getting one shot per week stalled after vaccine eligibility opened to all adults in April and has continued to decline.