How bout this...
Say light magic matures faster than dark, and reaches its near maximum relatively quickly, but then takes a considerably longer time to reach its full and maximum magic capabilities. Dark magic, on the other hand, takes longer to mature, but in its higher tiers is far more powerful than that of light magic user. So a seemingly well experienced light mage could trump a dark mage of the same power, but once a dark mage has grown to full strength, it would take a fully maxed out light mage to have an equal fight.