Where is the pressure vacuum breaker typically located in a lawn irrigation system?
Some may not even have it. You mention a well. If the well also provides your drinking water, then you should have a vacuum break. But if the well is only for irrigation then you will likely not have a vacuum break or back flow preventer.


  Where is the pressure vacuum breaker (or other backflow device) typically installed in a lawn irrigation system?


Usually right after the main shut-off. So if you know where the shut-off is, the vacuum break should be right there.


  Does it matter, as long as it is before the first sprinkler head? Or does it have to be before all the valves?


It is usually between the valves and the main shut-off.


  Is it typically buried or is it usually above ground and elevated?


If it's a vacuum break (and it should be) then it needs to be above ground. If it's a back-flow preventer then it could be buried.


  Would one ever be right at the jet pump on home well systems?


It's more likely co-located with the main shut-off than the pump.


  Do they ever have pressure gauges attached to them?


Usually not.