Thats funny that you should bring up this subject. I want to buy the equipment and start my dog in it. I have hooked my dog Zues (Rotty/Border Collie X) to inner tubes and been pulled He's 59 lbs. He loved it, when I pulled out that harness he would run around in circles.
I don't think your dog is too small or old if he's like most Aussies he's probaly full of energy. but you'll have to build up his muscles so he doesn't pull one. If you need too.