Lightning hasn't had the experience of the Boss, but Lightning has undergone training (presumably), has had to take care of her sister after the passing of her parents (tragedy and responsibility), and frankly her attitude throughout 13 does not reflect on a mature twenty-one year old. Apparently the father passed when she was young, the mother passed when she was fifteen, which means she had six years to grow up, which Lightning apparently did not do even while getting some army training.
Lightning's not a kid, just an adult acting like one. At 21, clearly the Boss was 'just a kid', wasn't she? But she was planning raids and bombings and participated in many missions. Meanwhile Lightning can't handle her own sister and flops around on her major decisions in 13, one minute just deciding to sneak through to Eden and attack Sanctum on her own (not much of a plan), and at the drop of a hat changing her mind.
I'm not handwaving. Lightning's motivations are flimsy. There's logical reasoning to suggest she should be much more mature than she is at the beginning of 13. Her subsequent actions reflect an even more shaky, immature, and indecisive character whose actions are subject to whim more than conviction and commitment. If she had followed through on her crusade against Sanctum despite her personal revelations I would be more impressed, because that would show Lightning thought through what destroying Sanctum meant for Cocoon and understood consequences.
We ended up with a posing strong female character.