WSO Identity Server 5.0.0 does not start with java 8. Probably because the underlying equinox version doesn't support it.
My question is: Will WSO2 Identity Server 5.1.0 support java 8?
WSO2IS 5.0.0 only supports for Java 6 and 7. Any WSO2 product does not support for java8 now. But they are currently working on it. See here. It seems be java8 support would be available soon for Carbon kernel. Therefore most probably WSO2IS 5.1.0 would support Java8