[captainjack]

...So I figured why not...

The primary reason I see for "why not" is that you don't want your pup to develope bad habits. A couple of the things I've seen (and done my self) at hunt tests are...

Allowing the dog to drag you to the line,
having the dog mishandle the birds.

If your pup can heel to the line without pulling in a very distracting environment of duck calls, gun shots, and whistles, AND is at least "lead steady", AND is delivering the birds nicely to the line (preferably to hand), AND has routinely shown the marking skills required at the started level in training (with the distractions), then go for it and have fun!

You will find that age is not a factor as dogs mature and acquire the skills needed at different rates. I know of a dog that was MH at about 14 months. You will see plenty of older dogs that run before they are ready if you watch for a while.