Before he became President of the United States, Mr Barack Obama ruled the waves of Hawaii dressed in checked swimming shorts. His wife, First Lady Michelle Obama, went one better, doing headstands in a backyard without so much as breaking into sweat.

But those days are gone. As a tweet on the White House says, accompanied by the youthful exploits of the First Couple - now aged 53 and 51 respectively: "No one stays young and invincible forever. Make sure you #GetCovered by February 15."

That's a call to Americans, especially the younger ones, to sign up for Obamacare, Mr Obama's signature universal healthcare law officially known as the Affordable Care Act.

"#TBT to being a young invincible. But now it's time to stay healthy & #GetCovered by Feb 15," Mrs Obama tweeted on her own account, using the popular hashtag for "Throwback Thursday", a social media trend for nostalgic tweets and postings, The Guardian reported.

Feb 15 is the deadline for the second enrolment period of the health insurance marketplace, the centrepiece of Obamacare, the newspaper noted. The tweet includes a link to the government's health coverage website.

Vice-president Joe Biden, 73, also posted a black and white picture of his childhood self, Guardian said.

Whether young Americans sign up for coverage may make or break Obamacare, which the opposition Republican Party has vowed to repeal. That's because younger clients' insurance premiums can help cover the medical claims for older ones.

Just over a quarter of those signing up to federal and state exchanges are aged between 18 and 34, the Guardian said.

The administration said earlier this week that 9.5 million people had signed up to receive health coverage through public marketplaces in 2015, and 4.5 million young Americans have gained health coverage through Obamacare since 2010.