Since and From are used differently. Since means from the reference point provided until now.
He borrowed my lawnmower and I have not seen him since.
This means that whenever the lawnmower was taken until the moment the sentence was written (the present moment), the author has not seen the person who took the lawnmower. It can also be written-
I have not seen him since he borrowed my lawnmower.
From establishes a time frame that does not need to be tied to the present moment. You would use it as follows.
I did not see him from June 14 until August 5.
The from establishes a start point and an end point for the time frame. These start and end points are often dates but they can be events too. You can even mix events and dates.
I did not see him from the time he borrowed my lawnmower until August the next summer.
As a few other posters have noted, from can be used to indicate a time frame extending to the present, if the sentence's tense implies that. We've been friends from childhood, can be understood because the tense implies we are still friends. This allows the sentence to be understood as, We've been friends from childhood until now. I would suggest, however, that the sentence is better written, We've been friends since childhood.