Forty is the old age of youth fifty the youth of old age.

Summary

This quote suggests that the concept of "old age" is subjective and relative. At forty, one is considered to be at the end of their youthful years, transitioning into a more mature phase of life, while at fifty, one can be seen as reclaiming some aspects of youth and vitality during the later stages of life. Essentially, the quote conveys that age is not solely defined by the number of years one has lived, but rather by the outlook, energy, and attitude one possesses.