under ground fence

I had an underground fence installed and went through all of the training as outlined by the "professional".  I have a german shorthair and a lab-mix and both had some difficulties adjusting but now are just fine in the yard as a whole.  There are still some safe spots that they don't visit but as a whole they will explore the entire yard and have their own little safe spots that they enjoy.  Honestly, it took nearly a year of exploring and my guidance to the safe areas before they really took to the whole yard (about a quarter acre).  My suggestion is to continue to train, praise, and let your dog know where it is safe to venture and be patient.

In my experience having had an underground fence in close to 20 years and having a half dozen different dogs during that time, they seem to get used to it pretty quickly.
But its possible things are different here as I have about 2 - 3 acres enclosed in the fence so they can roam a pretty big area till they get zapped.  It might be different on a city sized lot though. 
Can you turn down the sensitivity of the collar to give your dog the maximum room to run along?  On some you can decrease both the sensitivity and the zap power. 
Off topic, but I haven't even had the dog collar on my dog for 2 years and she never forgot where the boundary is, but noticed her wandering outside the fence a bit so put it back on to tune her up and keep her from disturbing pheasant and turkey nests.  She got zapped once and that's all it took. 
I agree with tstevens. I think if your dog has a large enough area and you hang in there she'll get used to it.

What is the appropriate age of a dog to start with the underground fence?

I recently installed a PetSafe underground fence around our 1 acre lot. I have an 8 month old pup.  I followed the break-in instructions in the manual, lead the dog to the flags marking the boundary wire, let him get "corrected" (what happened to "shocked"?) Only had to do this a couple of times. Then, you are supposed to lead him to the wire, leave him inside the safe area, then try to call him over to where he gets shocked - he only did that once, I couldn't get him to come close to the wire even waving his favorite treat.  There was a week or so when he only wanted to stay in the center area of the safe area - wouldn't go over towards the wire at all. But now he will go right up to the edge of the safe area and use the entire area.  This morning I was taking him out for a walk and had to take his transmitter collar off, then pick him up and carry him out of the lot.  On the return back, he had realized the collar was off and I was able to lead him back into our lot, but immediately put his collar on and made sure he knew what I was doing.

Over all, its been about 5 weeks since I installed it.

Nice topic.  Someday soon I may be exploring this option.  Any other folks out there with expierence with the fences?