Weight increases till about age 55 and then tends to lessen after that, probably due to loss of lean tissue, muscle mass, water and bone.
Bone loss is greater in women.
Total body fat remains constant, it just redistributes itself as more muscle tone is lost. Being lean is not an indication in living longer. Probably having your weight about in the middle range for your age is the best. The exceptions are death caused by cancer affecting the female organs, complications of pregnancy or childbirth and some relatively minor diseases. While muscle tone is lost as one gets older, apparently the heart does NOT weaken.