Hi Dave! Nice to see you on board.
This is what I was taught:
Most bergs surviving for up to a year after calving (breaking-off and floating away).
The majority of bergs in that area begin life at the edge of the glaciers of Greenland. They slowly drift out to sea and are initially carried southward by the current. Because currents are deflected to the right in the northern hemisphere; the ones hazardous to navigation are found mainly off the coasts of Greenland, Labrador and Newfoundland.
They usually calve in the summer months when the temperatures are higher. However, the majority of them are locked-in by pack ice during the following winter and they are effectively anchored during the cold months. They do not decrease much in volume during these months.
In spring, they continue their journey to the south.
The cold labrador current which carries them south also delays the melting process.
During the height of the ice season(April to June) it's not unusual for the biggest bergs penetrate as far south as 41N. I've never seen them that far south myself but that is what we were taught many years ago - long before the great interest in Titanic.
I've never seen the documentary you refer to. However, I deduce that a very large berg can only last for 2 years from it's date of birth so to speak, if it becomes grounded soon after calving and remains close to it's birthplace for another summer. Then the second summer, enough is melted off to allow it to float free. Since the water depth in the area governs the maximum draft of all floating ice and the draft of a berg is normally one or two times the height above the surface; we can roughly guess the maximum volume of any berg from a particular origin.
I guess what I'm trying to say is; once a berg floats free and starts it's journey south, only a decrease in water depth will delay it's journey. I do not think the age of the berg has anything to do with how far south it will travel.
The main criteria governing the life afloat of the berg is maximum possible draft afloat + underwater volume + temperature of the sea and air. en route. To a lesser extent, southerly gales may delay the southward passage but these are rare during the ice season. Most big winds come from the South West in January,February and March before the ice starts moving south.
No doubt you've read my article on the ice at the time. I believe that the labrador current was not particularly strong that year but spring came early in northern latitudes above Newfoundland. However if the Labrador current was a little colder than usual due to a greater than usual volume of melt-ice, it would sink sooner below the relatively warmer surface waters to the south. All this would have the effect of starting the ice on it's southward journey a few days earlier but slowing the shallower elements progressively as the surface current weakened. There had to be a point where that surface current came to a halt. I think that was between 41:30N and 42 30N - where the 'string' of pack ice was located between 12th and 16th April,1912. The bigger bergs seen to the south of that were the ones which floated free from their birth place with the maximum draft but with huge volume.