Generally, archery is available to youth. As Northshield is in the start of new kingdom process, the exact rules at any time will change. A parent should go with the young person to the archery field, until the young person is old enough and experienced enough to be very knowledgeable about the archery shooting practices and very responsible.
To participate, a youth needs a bow in good shape of the right strength and size. The bow should not be a compound bow. The arrows should have feather vanes not plastic vanes. The shafts of the arrows should be made of wood. The tips of the arrows should be target tips. The arrows should of appropriate size for the young person. Above all, young people need appropriate arm and finger protection. Even if the exact right equipment cannot be found, bring what you have and talk to the archery marshal. It is possible for the archery marshal, to waive that requirement for young people considering finding appropriately sized equipment for young people is difficult.
This page contains links that connect to sites over which neither the SCA nor the Kingdom of Northshield has any control.
I recently found a nice wooden youth bow for sale with 12 arrows for about a $100, available from Edwin Gilbert at http://www.horsebows.com/
Currently, there is also a nice wooden bow for $79 at http://www.woodbows.com/