I am thinking along the same lines for Boo i.e. fun training and outdoor obedience but at what age can you start? I have been told by friends that she is still too young at 5 months

5 months isn't to young. A lot of gundog trainers now hold puppy classes, so that the owner understands the things they should be doing through puppyhood to enhance instincts that will help with more advanced training at a later stage. One of these is keeping the dog close and walking the dog to heel without using a lead and by using the pups natural instinct to follow you everywhere.
So I would suggest you find a gundog trainer, have a chat, go along and see a lesson and make your own mind up.
The other thing about gundog pups, is mixing them with calm older dogs during the socialising period. This teaches them some manners as older dogs either ignore the pup or give it a telling off and use puppy classes for short play periods only. Doing this teaches the pup to be calm and to relax around other dogs.