(adapted from Likutay Halachos, Mincha 5) We said before that the development of the day relates to the development of creation. Now we will compare the development of one’s life to the cycle of the day. At first when one is young he grows in strength and intellect similar to the rising of the sun, which gets stronger and stronger with the passing of time. However when a person reaches around middle age he starts getting weaker, like the sun that starts setting from the middle of the day.
Even though Rebbe Nachman says that one day of serving Hashem as a youth is more important than many years of service when he is old (see Sefer Hamiddos Tshuva 73) still the older years are very precious.
In one’s older years one is supposed to come to his completion and the other side fights very hard at that time to overcome and stop him. Fighting at this stage is the Bechina of the Mincha prayer. The Yetzer Hara wants to stop a person when he starts getting weaker, but specifically then when the sun starts setting Hashem gave us the opportunity to daven Mincha and strengthen our faith in our hard times.