Crepe Myrtle Trees
My young Crepe Myrtle Trees have brown around the edge and also halfway along some of the leaves. They were covered with seeds and the nursery advised to cut them off for better blooms. I have already done this.
You mention your trees are young. If these are new plantings make sure the trees were not planted to deeply.
Also watering, new plantings will need plenty of water. Mulching with hardwood mulch will help with moisture retention. If the tree does not have enough water the tips of the leaves are the last to receive water and will dry out and turn brown.
I'm listing a few links for you to refresh you on the care requirements.