The New York Times health blog highlights the physical strength potential of women over 40:
"...Researchers have even measured the muscle strength of men and women in these communities [modern hunter-gatherers in Tanzania, Venezuela and Eastern Paraguay] and weighed the baskets and bundles carted around by them. Often, the scientists find, women in their 60s are as strong as women in their 20s. 'It's the women over 40 who are carrying the heavy loads,' said Dr. Hawkes."
Think about that on your next two-kettlebell farmer walk.