According to page 32 of op 6b they should give you 5 points for your relatives in Canada (http://www.cic.gc.ca/ENGLISH/RESOURCES/manuals/op/op06b-eng.pdf).
Relatives in Canada:
Award five points if the applicant or accompanying spouse or common- law partner has a relative (parent, grandparent, child, grandchild, child of a parent, child of a grandparent, or grandchild of a parent) who is residing in Canada and is a Canadian citizen or permanent resident.
You can email them and mention this part of the operation manual which clearly puts no age restriction for your child.
Good luck!