Actually I do startle easily. Cars backfiring, gunshots, any loud bang or immediate danger, I jump like anyone else. When I lived with my parents my father dealt drugs as well as used them and there would be people in and out of the house at all hours. So anytime I hear the creak of the floor, I wake up. Same with the click of a door, I'm up. Even in a dead sleep if I hear a voice I don't recognise it wakes me.
I also don't have that twilight sleep anymore. When I wake up now, I'm usually standing beside the bed before I finish blinking the sleep out of my eyes.
But I don't react to 'disasters' like most people do. When we used to live in Maryland we were hit by a hurricane a few times, and each time I would just calmly go look and see the damage, and either fix it or do a patch job until it could be fixed, I never freak out. Inever start crying, and I never played the victim well. In fact I'm usually the first one to say "I'm fine."
When people die I say "That sucks." and go back to what I was doing. When I was told I had a large tumor on my spinal cord which was paralyzing me. I started cracking jokes because my mom was with me and she was freaking out.
When something horrible happens I'm always the calmest person in the room. But if you just talking about a 'loud noises startle' then, yeah, I do that. In spades.