Hi David, should the dealer have noticed an impending head gasket failure? No, it is not something that can be seen in advance but they could have let you know of a water leak which could of caused the problem. Have you been checking your levels weekly? If so have you noticed a loss of water? If not then anybody would not be able to tell.
Is the vehicle a bit young for this? Nope, most h/gasket failures can be traced to low water level at some time which has caused localised hotspots in the head which damages the gasket, this then causes more problems until the gasket fails completely.
The easiest way to look after the engine is to check the levels regularly and if the headgasket has failed try to get the garage to find a cause, when they do it, get them to change the thermostat as it can be damaged by very high temps or could have been the original cause. Also check for cooling fan operation and leaking water pump+leaks from hoses, radiator and heater system. Good luck.