I have always started pups with easy trails of about 10m in a straight line,around 3/4 months old,only do perhaps a couple a month and then step up the difficulty as you go,put in bends and right angles over greater distances in differing cover,then when the dog is more advanced I start leaving gaps and going over water,without blood too
Older dogs can be trained to do this but I've found your better with pups to end up with an absolute cracker of a dog,note I'm not saying an older dog won't make a good trailing dog but they may be more interested in scents that they are used to already
I prefer pups,mold them into what you want easier