G-d told Moses, "From 25 years old and upwards the Levites must enter the service."
The Levites were given the responsibility of carrying the Tabernacle and its vessels through the desert. G-d led the Jews through the desert with the Tabernacle--in which His presence was openly manifest--in order to subdue the forces of evil, which the uninhabited, uncultivated, desolate, and dangerous desert embodied.
Similarly, our own environment can sometimes seem like a spiritual "desert," devoid of G-dliness. In fact, it may sometimes seem to us that we ourselves have become a "desert"--i.e., that we have developed habits that are contrary to our task of spreading Divine consciousness. How can we possibly hope to change ourselves (let alone the world around us) when these habits and modes of behavior have already become ingrained within us?
Here, we can learn from the Levites. They did not even begin training for their occupation until they were 25 years old, yet they were nevertheless empowered to begin their holy service at this age. G-d gives us, too, the ability to remake ourselves and begin new directions in life, even though we may feel unqualified, unprepared, or unworthy to carry them out. All we have to do is make use of His assistance.