Maturity and selfishness is two different things but Maturity and Selflessness is pretty much in the same ball park
let me explain the first point:
Being mature and selfish has different connotations. You can be a selfless person that shares alot, lets others shine, help a brother out etc...but at the same time always finding yourself in situations where u put yourself first. Thats where maturity comes in understanding where to do what, the art of seeing things before they happen, and becoming a planner. Some things in life cause us to become selfish.(me time, taking personal days at work) so that we can even be better people in general where in turn we become selfless naturally (taking more tasks at the job for others who need a day off, being in a better mood, leaving room to helping others) .
Now my second point: When you practice Selflessness, you are basically intertwined with a mature code of conduct. Selflessness is a practice of sacrifice, allowing yourself to take the back seat and consistently taking less shine, and being ok and living with it....it is an altruistic standpoint, and outlook of life, which basically says put others needs above your own. Only mature people can live with consistently putting others needs above themselves, and being genuinely ok with making those decisions....its the same reason Tom Brady takes less money to keep his team a contender year after year. He is thinking Im rich anyways, let my teamates eat, so we can ALL win on and OFF the field....maturity is about seeing things in different ways, to get the same conventional result.....you can be selfish and be mature, but you NEED to be MATURE to be Selfless.