13 has been accepted in the past. I understand that it is based on site insurance which previously required the age to be 13, but was revised to 12 on the basis that a 13th birthday would include some classmates/friends still aged 12.
My nephew was at last years NvS, and in my opinion was not the youngest that I saw playing, but I'm no judge of age.
So its basically call the shop and confirm with them then fill in the parent / guardian consent in the back of the book.