Why does the doctor claim to have "so well selected the subjects of my experiment"?

The characters were so well selected because they spent
their lives in pursuit of unrealistic expectations and life's foolish pleasures of
vanity and physical pleasure. Mr. Medbourne is a merchant who was once wealthy, but has
since losthis wealth. Colonel Killigrew is now a decaying old man who was once a
ladies’man in his frivolous youth. Mr. Gascoigne is a now unknown but was once a
notorious and unprincipled politician. The Widow Wycherly is now a shriveled old woman
who was once a young woman of loose morals. During the experiment, they cavort and caper
and flirt and, ultimately, spill the magical water. Rather than returning to youth
accompanied by the wisdom of the aged, they return to youth and repeat their same poor
behaviors.