The photo is credited to the "Davison Community Schools", so it's most likely taken from 1972.
Here are some more pictures of a younger Moore. http://petercavanaugh.wordpress.com/2009/09/28/mi...

Also, look at this comparison picture: http://yfrog.com/f/6fmoorejj/

Look at their noses, the spacing of the eyes, but most importantly look at their teeth.
Look at Moore's teeth from the early 70s to today, they haven't changed. Look at the guy in the video and compare them.

---
As for "there’s no way to tell other than to take the YouTube uploaders’ word for it"
1. Ask the uploader for clarification.
2. Ask Moore himself.

I don't think this is really Michael Moore. I think the uploader of the video referred to the guy as a young Michael Moore, much like someone might refer to a young inventor as "a young Thomas Edison".

Here is a picture of Moore from the 1970s. He was husky back then. http://www.usatoday.com/life/movies/news/2004-06-...

I don't think this is real Michael Moore. I think the uploader of the video referred to the guy as a young Michael Moore, much like someone might refer to a young inventor as "a young Thomas Edison".

Here is a picture of Moore from the 70s.
http://www.usatoday.com/life/movies/news/2004-06-...