Upper limit - 20 minutes !

Shown a 50 minutes documentary today. I notice that children (age 12 -13 yrs) get distracted after about 15-20 minutes. They chat or needle some one or drift. Thus even a well made documentary on an important issue - directly related to their lessons becomes in-effective. I wonder if the 15-20 minute upper limit of their focus is shaped by the TV commercial breaks.

• Is it too early to expect 40 minutes of attention span for these 12-13 year-olds ?
  
• Or is it already late to expect 40 minutes of attention span for these 12-13 year-olds ?