sorry i think we are at either end of a different stick here . Gundogs actually are actually trained into the dummies and they then retrieve it back to the handler ! That is not really natural behaviour at all for a dog .
Fully agree with all the above though , however we are talking deer and the dog ( well all i have had or worked with ) will take interest in a trail of blood and are very keen to get a hold of a dead deer . If not it takes only a few seconds of encouragement . Now they just need practice not training in the task of tracking . Its then down to amount of practice .
I am into number 3 deer dog , none of them needed " training " in that task but every one of them needed to get a handle on retrieving birds and other small game to hand . Because deer are the natural prey of dogs I just "facilitated "
To the question " can a 2 year old lab be taught to track ?" I would say heck no it already can it just needs the owner to get out there and give it opportunity and practice . Sure there are nice to have actions that might need std Pavlovian training like recall, sitting and staying etc
You know maybe the best dog i have used successfully was my wildfowling dog , I lost a big sika stag into some really thick cover , the dog found it very, very easy ! Indeed i got that look from him the one i recognised as " you couldn't even find that one dumb ass ! "