Baldness does not usually manifest until the male has become a successful provider. It is equivalent to the silver-back phenomenon in gorillas.
A male who reaches the age to display his scalp has proven that he has demonstrated intelligence, skill, or luck to survive the youthful indiscretions (like wars or communicable diseases) which carry off so many males before they reach their optimal desirability as mating partners. We also grow that hair on our backs at this stage of life; this provides something for the offspring to hold onto while the tribe is on the move.