Java 7 was released in July 2011, it hasn't received public updates since April 2015, Java 8 was released in March 2014 and Java 9 is scheduled to be released in July 2017.
The last public release of JDK 7 by Oracle contains a large number of known security vulnerabilities and Java 8 introduces a number of
compelling features and we will soon have to support Java 9 so it would be good to drop support for Java 7 in 2017. The actual timing would depend on when we release the next major release of Kafka.
More details can be found in the KIP: