It depends on the dog, first let me say, there are alot of really low impact things you can do with young puppies, however, I do like to get a really SOLID obedience foundation on them before I start any agility..That includes sit, down, stay and a ROCK solid recall..
I also never do full height jumping until around 18mths-2years of age and only after I have my dog xrayed from stem to stern, including, hips, knees, elbows, shoulder, spine,,I want a dog who is in excellent phsyical condition structurally..I swim my dogs ALOT, good way to keep up muscle tone..
I would start a dog your age in a beginner class, only with good those basics of obedience solid and after a full physical checkup ..
Diane