Close adult friendships, gender and the life cycle
Brian Joseph Gillespie, Janet Lever, David Frederick, and Tracy Royce. Journal of Social and Personal Relationships. 2014.
People who have more friends are fitter physically and mentally and live longer. For older people contact with friends is more important than contact with adult children. Female friendships are "face to face" while men are "side to side" - women build friendships while men do things together. But you have to be careful measuring male relationships using female standards of behaviour.
This area hasn't been researched much.