well with the empire going out to the outer rim planets they had more places to search so maybe he was being silent for the time until luke came to ask him for help don't know if he could have trained luke while he was young because uncle would not allow because of what happened to his father that's my opinion.
Well, if Obi-Wan had been training Luke from early ages either:
A. Obi-Wan would have done the same thing with Anakin, and Luke would totally flip out and kill Obi-Wan.
B. Palpatine or Vader would sense such a great disturbance in the force and they would bomb the Lars homestead earlier.
C. Luke would complain to Obi-Wan about how the lightsabre destroyed the power converters he just got.