The hardships and tests we face - God willing - refine us not only as human beings but increase our spiritual understanding and capacity to behold the divine. This is one reason why human being agreed to accept the responsibility of free will - with it comes a risk (hellfire) but also great potential.
Ideally the main goal of the material existence (as I understand it) is for human beings to deepen their inherent spiritual understanding as much as possible. A child and an adult may both go to jannah; however, depending on their experiences (as not everyone grows throughout a long lifespan), they may end up with a different inherent capacity to perceive the divine there. (One might equate that with "higher in jennah")