AMES, Iowa—Bernie Sanders’s recent heart attack and hospitalization are bringing to the forefront an issue the Democratic party’s three leading presidential candidates have actively tried to minimize: their age.

At 78, the Vermont senator is the oldest in the party’s field. Former Vice President Joe Biden is 76, and Massachusetts Sen. Elizabeth Warren is 70. All three—who have consistently occupied the trio of top slots in public polls of the Democratic contest—are much older than the median age of about 55 years for a U.S. president at inauguration. They are competing to face President Trump, who is 73.

The candidates’ ages, which have bubbled up as an issue for some throughout the primary, is now the source of renewed concern, according to interviews with strategists, Democratic county chairs in early states and voters in Iowa, which hosts the nation’s first nominating contest.

“It factors into the discussions that people have locally,” said Peter Leo, chairman of Carroll County Democrats in Iowa. “I don’t know if it’s anyone’s No. 1 concern, but there is some trepidation.”

Mr. Leo said some activists and likely caucus-goers he has talked to have mentioned the importance of having a younger vice presidential candidate on the ballot in case Mr. Biden, Ms. Warren or Mr. Sanders is the nominee.