i give them the chance to start from 11 month but it depends on the dog.
chance means i take them to a hole and about 2-5 meter far away i set him on the ground and check what he is doing.
is he useing his nose or what.
if he is playing around good but back in car and some weeks later again.
when he starts great!
when you know about (your) dogs you see it (mostly). not how good the dog wil be but is he ready for a try out.
rats they do mostly as 8-10 weeks puppy already but i am talkin about patterdales. maybe jacks are different?