As they age, women tend to become psychologically more independent, for a variety of reasons, while men become more dependent, particularly when they retire and spend more time at home (the traditional woman’s domain). As the men of the Grant Study and their wives became more equal, and probably shared more interests by virtue of being together more, they probably became more companionable. Vaillant refers to “hormonal changes that ‘feminize’ husbands and ‘masculinize’ wives,” but I don’t think it’s necessary to invoke them to explain the growing closeness. He also believes the “empty nest is often more of a blessing than a burden,” and I think he’s right in that.
A more speculative possibility: it seems to me that old age takes many men almost by surprise; it sneaks up on them, and is all the more disturbing for that. In contrast, women are all too aware of aging, starting with their first gray hair or wrinkle. By the time they’re in their fifties, they’re well accustomed to the losses that come with age. That may make them better able to help and support their husbands as the men find that having been a master of the universe is no protection against old age.