NEW YORK » Another 17 of America’s richest people, including Facebook CEO Mark Zuckerberg, junk bond pioneer Michael Milken and AOL co-founder and former Hawaii resident Steve Case, have promised to give away most of their wealth.

At 26, Zuckerberg has put himself on the map not only as one of the world’s youngest billionaires, but also as a prominent newcomer to the world of philanthropy. Earlier this year he pledged $100 million over five years to the Newark, N.J., school system. Now he’s in the company of media titans Carl Icahn, 74, Barry Diller, 68, and others who have joined Giving Pledge, an effort led by Microsoft founder Bill Gates and investor Warren Buffett to commit the country’s wealthiest people to step up their charitable donations.

The group has signed up 57 people and their families since launching the campaign in June. The list also includes New York Mayor Michael Bloomberg, CNN founder Ted Turner and film director George Lucas. But Zuckerberg and Facebook co-founder Dustin Moskowitz, 26, are the two youngest.

"People wait until late in their career to give back. But why wait when there is so much to be done?" Zuckerberg said in a statement. "With a generation of younger folks who have thrived on the success of their companies, there is a big opportunity for many of us to give back earlier in our lifetime and see the impact of our philanthropic efforts."

Though the pledge is not a formal contract, those who make it are committing to give away at least half of their wealth to philanthropic causes either in their lifetime or after they die.

By pushing the age for substantial charitable giving lower, Zuckerberg is challenging today’s generation of tech entrepreneurs to think not just about how they will make their next million — or billion — but how they will give it away.

"Younger donors think longer term because they will be giving longer term," said Jason Franklin, a professor at New York University and the executive director of the nonprofit Bolder Giving, which encourages people to give.

"Giving when you are older, you give so your children will have a better world. Giving when you are in your 20s, you give so you will have a better world."