You can start training agility whenever you want. Many people start very young, before the dog even has basic obedience just to get them used to all the equipment. I just started my dog because he's my first and I wanted to make sure obedience was rock solid before we went into the agility ring.
If your dog listens to you, works well with you, doesn't get distracted and if he does you can easily call her back over to you, it won't be an issue. In my class we do have a few dogs that can have their moments and it just makes me (and other handlers) mad because they're wasting time running around chasing the dog.
People that have done both agility and Schutzhund might be able to help you more, but as someone that is in AKC obedience and now starting agility I have noticed that his focus is a little less overall since we've started. He knows he has the freedom to do a lot more and I really have to make sure to direct him in the proper direction before he just does things on his own (loves jumping and climbing and anything else that has to do with agility). It probably doesn't help that we train on the same field for both, but its just something I have to work through with him.
CD RE TC HIC 7/10