These guys have pretty much nailed it, so I will only add that I've found knowing that being confident yet not cocky and friendly yet firm has gotten me far as younger umpire and football official. At times, it is not easy and you will have coaches that challenge you soley due to your age. Have the confidence to know you can do a good job and pay the bozos no mind.
For what it's worth, my first ejection was at the age of 15. I ran a coach that was in his 40's for getting in my face and continually arguing a call. If you haven't ejected an older coach yet, it will get easier with time.
