By the way, the letter in Fellrunner seems to refer to the Wincle Trout Run.
http://www.wincle.org.uk/pdfs/Wincle..._form_2012.pdf
The form seems very clear to me.

To do the Senior Race you need to be 16 on 1/1/12

To do the Junior Race you need to be 10-15 on 1/1/12

and as there are "mini races for U10s" I can't see how anyone is turned away, they just guide them to the appropriate race for their age.