I have been concerned about this issue for some time. There is not a lot of literature about it but seems there needs to be. One boy I coached had a concussion (age 10) while heading a punted ball. Some of the literature suggests avoiding headers until boys are 12 and girls are 14.
Here is an article today talking about the risk of head injuries for girl soccer players in particular. http://rockcenter.msnbc.msn.com/_news/2012/05/09/11604307-concussion-crisis-growing-in-girls-soccer?lite&google_editors_picks=true.
I think there needs to be some exploration about the teaching and use of headers for boys under 12 and girls under 14. Most of the literature focuses on contact with other players or the post, but it seems there is a risk with heading that needs to be addressed.
I did a little more looking and there have been some research on the link between heading and concussions. Not tests had been conducted on youth, but it was recommended that 10 and under youth should not head the ball. Here are a couple of helpful links:
I have looked at headgear to wear and even bought one (looks like a sweatband – you can find it at amazon).. “Real” soccer players look down on this but maybe we can reverse the peer pressure! Here are some examples of soccer protective head gear:
Article talking to about the effectiveness of three different protective headgears used for soccer.