How fast does an average person run per 100 meters ?

What's an average person? there's way too many factors in it to give an average time! age, weight, sex, build, fitness, bodyfat ratio.

I'm sorry it's not a very helpful answer but sadly the question is just too broad... if it helps any then I can manage approx 13s... I'm 5'10", 12st10lb, and oyu can gauge my fitness levels form the PB's in my profile (though ignore the 5k!)

Depends whether chased by an average-sized dog

I don't think there really is "average" in any meaningful sense; it depends so much on fitness level, natural speed, experience, etc etc

I mean - average slim and light person.

Average slim person who did a lot of exercise in high school and still does exercise in college.

How fast does said average person run at the moment, and how fast would this person like to become in the future? How many miles per week does this person run, and how long has he / she been running for?

At a very rough estimate, I would say anything from 7 to 11 mins per mile. For 100m sprints, I wouldn't really know I do approx 7:40 depending on distance, conditions and so on.

If we're talking about a 100m sprint, I don't think that fitness would play a great part in it. Even untrained people would manage to put in an "all-out" effort over that distance, and maybe even a little further.
So as the distance moves up, I think we'd find that the "average" person would occupy a different spot in the over-all standings.

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GONE RUNNING

If you are thinking about your own performance then I wouldn't obsess about average people and about stats - forget em.

However, if you want to look at some stats for average people then I recommend taking a look at the BPTT results for 5km - take a look especially at "Unattached" runners - this will give you a good spread and will break it down by sex and age (but not weight - not yet, anyway...)