Adolescence is such a complicated time. Our teens’ bodies are making radical changes and their brains are maturing while busy pruning unused synapses. At the same time there is enormous social upheaval made more potent and dramatic because of the impact of social media. 

Now brain scientists are parsing out the details of when empathy is developed stating that affective and cognitive empathy develop not just in childhood, but in the teenage years, first in girls and then in boys.
This information helps explains the cruelty in middle school when so many teenagers are indifferent to the suffering of their peers on Facebook,, etc. 

Read, Teens Are Still Developing Empathy Skills by Sue Shellenbarger in The Wall Street Journal

to learn more.