YES! By GOD! It's not just the decibel level, it's the PITCH that will gradually affect your hearing. You should ALWAYS wear either muffs or plugs, indoors? BOTH! If you're firing under any type of structure, even a tarp, you need them especially. If the noise can reflect off anything nearby you should wear them, and you should wear something anyway.
Working at a range with kids on it, we can't stress this enough. I personally always wear unless I am hunting, I'd rather not deal with them for just the one shot, unless I plan to shoot more than 3 or 4 times, like duck hunting I suppose.
You might feel okay right away, but even then sometimes the loss can be unseen. You ear heals well, but over time it doesn't heal 100% at all. If you talk to some older guys that have been shooting for awhile, they're deaf as hell.
Anyone under the age of 18 should wear at all times, everyone else its their choice. Children need to because they are still developing, and you don't want to get any hearing damage in that stage.