Read the passage given below and pick the option that best fits the question that follows:
The art of growing old is one which the passage of time has forced upon my attention. Psychologically there are two dangers to be guarded against in old age. One of these is undue absorption in the past. It does not do to live in memories, in regrets for the good old days. One's thoughts must be directed to the future. This is not always easy; one's past is a gradually increasing weight. The other thing to be avoided is clinging to youth on the hope of sucking vigour from its vitality.
"By the art of growing old", the author means ______.