This is a story I read in the early 1970s, quite possibly in Boys' Life (though maybe an old issue). The main character is a boy who is in a wheelchair and terminally ill. He (somehow) is out in the wilderness when a UFO lands and is surrounded by Earth (US) military. Again, somehow, he is taken aboard the UFO, and learns that the UFO is actually from the future, and are human. After he leaves, he realizes that the future humans weren't talking in generalities when they said things like "You will live" (which he had interpreted as "there's no nuclear war coming") - they had cured him (and perhaps were his direct descendants).
This is "The Shining Ones" by Ben Bova.
A shortened version of the story appeared in the February 1974 issue of Boys' Life. It can be read on Google Books. The story begins on page 28.
The full version of the story first appeared in Notes to a Science Fiction Writer by Ben Bova. It can be borrowed from the Internet Archive.
A brief snippet from toward the end:
"Your heart stopped beating," said the first alien. "We also found a few flaws in your body chemistry, which we corrected. But we took no action to prolong your life span. You will live some 80 to 100 years, just as the others of your race do. The history of your times has shown us that."
Eighty to 100 years! Johnny was thunderstruck. The "other flaws in body chemistry" that they had fixed—they had cured the leukemia!