A lot of good advice so far. I do not like to run them until most of the bad habits for the level you are going to run have been taken care of in training. It is easy to reward bad habits in a HT if the dog is not ready and few juniors have ever seen the excitement that they will encounter in their first HT which will only add to the problem(s) that may exist.
JMHO, but if your pup has been FF, delivers to hand, has been on live birds, has fair line manners, and can consistently do nice simple single marks on land and water, your dog should be OK in a HT. I would never go as far as to say any certain age. I have run dogs in juniors as young as 7 months and as old as 7 years, different dogs, different schedules.
A couple of things I mention above are not absolute. A lot of trainers do not have the means to shoot flyers in training and many a dog has seen its first flyer in a junior test, but I would like to at least have the dog working with dead birds. You do not want to introduce a dog to birds in a hunt test or you will find your self chasing a dog that is trying to eat a bird. When I say fair line manners for a junior in its first HT I mean you can walk the dog from the holding blind to the line with all four feet on the ground, not pulling you to the line. The dog does not have to be steady at this point, but you should be able to easily restrain him on the line. You can use a collar in juniors and you really should. I have seen a fair number of dogs that the owner has sure would sit still that broke in the excitement (including one or two of my own)