While it doesn't explicitly depict people getting younger, the novel features a time traveler who journeys into the far future and encounters a society called the Eloi. The Eloi are a peaceful, childlike race who have devolved physically and mentally due to the passage of time. Their society is stagnant, and they seem to have no concept of history or progress.
The time traveler also encounters the Morlocks, a subterranean race that has evolved into predatory creatures who feed on the Eloi. This suggests a possible evolutionary path where humanity regresses in a backwards progression of time.
While the novel doesn't literally describe people physically getting younger, it presents the concept of time reversing through the evolution of the Eloi and Morlocks, showcasing a society that has regressed in time.