Spider Baby (1964)
Spider Baby is a 1964 black comedy horror film, written and directed by Jack Hill. It stars Lon Chaney, Jr. as Bruno, the chauffeur and caretaker of three orphaned siblings who suffer from "Merrye Syndrome", which causes them to mentally, socially, and physically regress backwards down the evolutionary ladder starting in early puberty. Sid Haig, Jill Banner, Carol Ohmart, Quinn Redeker, Mary Mitchel, and Karl Schanzer also star.
More at Wikipedia.