There is more to disparity of force than just age, sex or weight differential. I'm a big guy, with two bad knees. Maybe you have a heart condition or bleed easily. In the end, self defense is an affirmative defense where you must convince that a reasonable person, knowing what you knew at the time, would feel your actions are justified.
If I'm jumped from behind, knocked to the ground and in the process of being beaten down, I might just find that reasonable.

