A lightweight bird gun around the 6 lb. range is a special treat, but I don't think it is an absolute must-have necessity. Carrying an extra pound or so really shouldn't make that much difference over the course of a day to a grown man in reasonably good physical condition.
Getting older certainly does make a difference, but proper conditioning, or the lack thereof probably pays a bigger role. I noticed a long time ago the level of fatigue I had after a long day of hunting depended upon what sort of shape I was in. Specifically, it didn't matter how much I walked at work or around home, as much as HOW and WHERE I walked. We use a much different set of muscles when walking in the woods or brushy fields than we do when walking on smooth level ground or pavement. Being out in the woods working with a chain saw cutting firewood before the hunting season will naturally get you in better shape for hunting than any amount of walking on flat floors at work, or even on a nice manicured golf course or lawn.
Very soon after you stop using the muscles you used during hunting season to drag heavier boots through brush and tangled grasses, you lose strength in those muscles you stopped using. When your feet and leg muscles no longer have to accommodate uneven ground and stepping over tree limbs and logs, you soon lose that special sort of conditioning that hunting demands. Use it or lose it. And no amount of walking on a treadmill at a gym is going to get it back. Walking and jogging may improve your cardiovascular system, but you still won't be fully prepared for climbing hills in the woods and busting through brush. Same goes for hiking through deep snow, or swampy ground. Until you are actually in shape for the type of terrain you encounter while hunting, your tail will still be dragging after the end of a long day, even with what you think is a magic under 6 lb. shotgun.
And if you want to make a 7 1/2 lb. gun seem light, all you need to do is carry something heavier for a while. It really works.
