You are not alone, everyone feels like that from time to time - when it's time for a break - mentally. Try and have a break away from your course work - a day, an afternoon or even an hour or two where you do not give it a thought. Sometimes just sleeping on it allows you to have fresh thoughts in the morning.
Have a look at these links and see if they help.
http://www.livestrong.com/article/15...at-7-16-years/
http://www.ehow.com/about_6059546_ph...-12-years.html
http://www.merckmanuals.com/home/au/...68/ch268d.html
The last link goes upto twelve.