This 20-year-old wants to interview every WWII combat veteran he can before it's too late

Rishi Sharma with Harold Nelson, 103. Nelson served with the 3rd Infantry Division, was wounded twice and was put up for the Silver Star medal.

(CNN)Ever since he was little, Rishi Sharma has enjoyed learning about the Second World War. Now he's taken his passion a big step further.

The 20-year-old from Agoura, California, is on a mission, and he's got a time crunch. He's trying to interview as many living World War II combat veterans as he can, to document their stories before they are lost forever.
In the past four years he has traveled to 45 states and Canada to interview more than 870 veterans.
But Sharma, who considers these vets his real-life history book, is still looking for more.
    He wants to preserve their stories for future generations. And he's grateful to them for their sacrifice.
    "They've given us the world that we have," he said. "It's truly amazing."
    Carl Patrinos, 99, fought with the 32nd Infantry Division in the Pacific.

    One vet at a time

    Sharma, who doesn't come from a military family, has bitten off a big job.
    According to the US Department of Veterans Affairs, 558,000 of the 16 million Americans who served in World War II were still alive in 2017.
    But the youngest of them are in their late eighties, and some are more than 100 years old. The VA estimates an average of 362 of them die each day.
    Sharma was a sophomore in high school when he began what he calls his "mission." He heard about a decorated veteran, Lyle Bouck, whose tiny unit had held off a much larger German battalion during the Battle of the Bulge.
    Sharma tracked Bouck down and interviewed him.
    Then he took it a step further.
    He began biking to retirement homes to get to know the veterans in his community. Sometimes it was as simple as showing up and asking to speak to them. Many aging veterans don't get many visitors and are eager to share their stories, he said.
    Sharma records the interviews on video and burns them to DVDs, which he gives to the veterans. Some of them want to make their stories public, while others prefer to keep them within their family to help their descendants understand what they went through on the battlefield.
    He also has begun posting the interviews to his YouTube channel.