"To me, he was a star," Sheila Nevins, president of HBO Documentary Films, told PEOPLE on Sunday morning, calling news of Sam's death "devastating."
Last October, the cable network aired its film Life According to Sam, and a determined Sam personally attended its premiere showing in New York City – but watched his story unfold onscreen all by himself in an adjoining room.
Afterward, a silent Sam walked into the main screening room and unfolded a napkin before him, on which he had written: "AWESOME!"
"We were so relieved," said Nevins.
At the start of the documentary, Sam, a precocious student and devoted sports fan, said: "I didn't put myself in front of you to have you feel bad for me. I put myself in front of you to let you know you don't need to feel bad for me. I want you to know me. This is my life, and progeria is part of it. It's not a major part of it, but it is part of it."
Also mourning Sam is New England Patriots owner Robert Kraft, who said he had invited the teen to be the team's honorary captain for Saturday night's playoff game and was looking forward to spending time with him, reports the Associated Press.
"I loved Sam Berns and am richer for having known him," Kraft said in a statement Saturday.
In 2003, PEOPLE profiled Sam, then a kindergartener. "Sam was devastatingly rare: He's among the estimated 1 in 4 million children born with progeria, an incurable genetic disorder that rapidly transforms them into wizened old men and women.
HBO Documentary Films
So did Sam, who possessed the most winning personality one could ever hope to meet.
His parents founded the nonprofit foundation after encountering a lack of information and research on the fatal condition, whose victims live an average of 13 years. Sam was diagnosed at 22 months.
Sam's motto was "nothing is impossible." Bolstered by a daily shot of human growth hormone, which helped help him get bigger and stronger, Sam underwent physical therapy to work on flexibility and range of motion.
"The kids play chase all the time, and he keeps up," kindergarten teacher Laurie Waterman, who placed Sam in the top reading group, told PEOPLE in 2003.
After school he played youth baseball. "I can really hit," boasted the remarkable Sam Berns. "I practice at home – I'm really good."