You can start the obstacles at any age if you make them very easy and go slowly. So, stepping over the "jump" ( a broomstick lying on the ground), and cuing it using your cue word to jump. Get a little tunnel and have him go through it. Hold a hula hoop on the ground, and ask him to step through it. Set up a tiny dog walk, just 3" off the ground, and have him walk it. You can make a little wobble board, again, just a tiny bit off the ground, so that he gets the idea that he can balance on something.
Just make sure, of course, that everything is very easy, very safe, and that he gets tons of treats and praise, and don't ask him to do these things rapidly, only at a walk. If he makes any kind of mistake, ignore it. If he tries but doesn't quite succeed, praise the trying like crazy. This way it is a game he always wins.
Many people start training their agility puppies at a very early age like this, and so do people who are training service dogs. I was visiting a litter of aussie puppies the other day. 8 weeks old and they all knew how to go through the tunnel when told to, and to step over a stick and walk through a hoop. They also all lined up and sat waiting for their food. It was so cute, and they loved it. I could see how confident it made them to be succeeding at these things and getting rewarded.
