Age to start

if you are lucky and get some really well breed hog dog pups then you won't have to train as hard or often...but like most have already mention mock hunts and staging different scenarios will help quite a bit...have trailing and then winding mock hunts...bay pen is also good in between mock hunts...

dog handling is key to bringing out the best from your pups...like you already said don't train too often and don't wear the pups out by too long of sessions or they can possibly lose interest due to burn out...
also taking them out and try to turn them out on fresh hog sign so the chance of them finding and baying a hog goes up...also the chance of striking trash is a lot lower...walk slow and stop and let them work out a track and wait on them to get back if they lose it...sometimes you will have to lead the pups in the direction where you think the hogs could be laid up...let your imagination be your guide...it is all about the dogs...your job is to bring out the best in them by leading and guiding encouraging them on or scolding when they trash...if you have good quality pups you will see improvement by leaps and bounds...