was so low simply because of a very high infant and child mortality. People in the past did not age any faster than we do now, although they certainly didn't have anything like the modern medical care we have today. Even something as simple as aspirin makes a difference. And just understanding the germ theory of disease helps. Heck, if any of us here were transported back into the past, we'd do rather well in maintaining health just because of knowing such things.
Back to the weight loss issue. It's my understanding that even if a person can't maintain an "ideal" weight all the time, the periods of lower weight are beneficial.
Another issue is that we all live a vastly more sedentary life than our ancestors did, and we have access to food in an amazing way. In this country, while people do go hungry, actual starvation is almost unheard of. Even though a lot still needs to be done, food banks and kitchens that feed the homeless (I volunteer at one) make a huge difference in this area.