I started my lab/weim mix, Cash, at about 12 weeks, using a chunk of deer hide and a piece of stew meat on a string. I started out leaving short trails in the yard. He took to it right away. I made sure to give him lots of praise and belly rubs when he found it.
That first season, when he was about 5 months, I let him track a doe I shot, although she fell within sight. He went right to her. The next year, up in the mountains we tracked a rifle shot deer for a friend. We found that deer after a 1/4 mile track down in a steep ravine. We never would have found it without Cash.
Cash was a great dog and had a natural point on birds. He was a handful when bloodtrailing, though, due to his size and strength. He'd yank you off your feet if you weren't paying attention.
Sadly, we lost him to some sort of lung cancer at the age of 5. I sure do miss my buddy.
Now, my tracking dog is a Russell terrier, Taku. He's every bit as good as Cash was, but I do have to help him over obstacles at times.