The Siege : 43 yr old as a Major General?

43 seems too young for someone to already be a General. That's how old Bruce Willis was when The Siege was released. I wonder why Bruce had to be a General and not a Colonel or something. Or, why they even went with Bruce Willis and not some older actor.

Bruce was probably inspired by starring with Keith David around that same time, since Keith made four-star by age 42 before sending Bruce to blow up an asteroid, so Bruce thought it was realistic.
Seriously though you're probably correct as even risers like MG Garrison or General McChrystal were in their mid-forties when they attained their second star. A career in specialized units and combat would make it plausible. Or, you know, WW2

That would be correct if you're assuming that the character is the same age as the actor.