Queen Elizabeth rules over a fraction of the territories and colonies and dominions that her father did; India split in '47, so there's a billion souls off of her plate right from the go.

World War II was so much bigger, more involved, and had so much more at stake than any conflict since then in terms of lives, damage, political change, etc. Hitler came within an ace of conquering Continental Europe totally in the fall of 1940, and England was the ONLY power that was actively opposing Germany with any real committment at that time. The British were fearful (and half-certain) that England was going to be invaded, this fear only slacked off in early '42. They had plans for anti-tank ditches in Picadilly square, dumping oil in the Channel and setting it aflame, etc. Couple this with the large losses the British Empire took from the Axis in every part of the world during those first few years, and you can begin to see how it would wear on a man. King George was the national symbol of England in a time of intense crisis. That's an enormous amount of strain.

Compare to this Elizabeth's reign; what's left of the British Empire is firmly allied with America almost unilaterally, there hasn't been a major conflict since Korea, and every military action since then that they've been wrapped up in has either been rebellion-based (Malaya), NATO-backed policing actions, and low-level dustups like the Falklands where there might be a lot of prestige at stake, but very few boots on the ground and no risk of serious harm should they be lost anyway.

Their Majesties refused to leave London during the Blitz, leaving them under the threat of bombardment like any other Londoner. In fact, a bomb did go off right outside their sitting room whilst they were in it. That would put a couple of years on anyone.