How much should an average 5.6 feet 15 yrs old weigh?

The answer varies greatly depending on a number of factors. These include gender, activity level, and whether or not that person is involved in sports. It matters because someone who is very active, plays a sport nearly everyday, etc, are going to have more muscle mass and thus weigh more. So the figures I'm about to offer are absolutely moot for muscled individuals. Males at 5 feet 6 inches:

Small frame- 136-142lbs

Medium frame- 139-151lbs

Large frame- 146-164lbs Females at 5 feet 6 inches: Small frame- 120-133lbs

Medium frame- 130-144lbs

Large frame- 140-159lbs Source-http://www.dietbites.com/Weight-Maintenance-Journal/Apr-07-2008.html