Dabbs Greer aka Old Tom Hanks
Watching this for the first time I note one of the rescueship crewmen is a character actor most anybody would recognize but never know by name. He had a long career, even including the part as the first man saved by George Reeves in the first episode of Superman.
Looking him up Dabbs Greer was born in 1917, making him 41 at the time of this film.
One of his last and probably his finest role was as the aged version of Tom Hanks in The Green Mile. I just think it's interesting to consider that when "It" was filmed, Hanks hadn't yet turned two.