I thought of this after watching it again.
Widmore: "are u here to kill me benjamin" Ben "we both know i cant do that"
I believe this is because Widmore was touched by Jacob. This is what happened to Richard. People touched by Jacob cannot be killed. For example Jack tried to kill himself in the finale of season 3 and was stopped because of the car crash. Locke tried to kill himself in the dharma hole and was stopped by walt. Maybe michael was touched as well which is why he couldn't kill himself. This doesn't explain why richard can't age though.
Just some thoughts anyway. Any opinions?