What lesson or lessons does the story teach?
Everyone can take different things from the story, but one possible lesson is the idea of being content or happy about one's situation in life, in this case particularly about the age. The old men become young and then compete for the beauty of the (now) young woman in the group. They are absolutely horrified when they grow old again and plan to seek the water for themselves because they cannot be happy with their current dilapidated state.
It was easy for the doctor to watch them and learn the lesson, but much more difficult for the subjects that actually felt the change and felt the vigor of youth and then desperately longed to continue the experience.