I don' think that the word "instinct" describes much more than our ignorance of the source of behaviors. Taking the examples of hunter/gatherer societies we have been able to observe, gathering usually provides the bulk of calories, and most gathering/processing operations need little athleticism while requiring a certain amount of dexterity, knowledge, and cooperation. There are many instances of this kind of participation, far past "reproductive age" still directly observable and indirectly through skeletal remains, teeth, and coprolites (fossilized or preserved feces).

The "harsh dangers of the wilds" are largely overblown in popular versions of hominid life. The bonded social group seems to go back a million years or more, and the most vulnerable members were always the infants and children, You need only see how social animals like elephants, bison, yaks, gorillas etc. use social power to defend their vulnerable young to visualize similar defense strategies.

Evolution does not seek to select the "best qualities or features", evolution does not know which features will prove to be necessary. As Darwin pointed out, "fitness" is not a measure of superiority or inferiority, but merely the fit between physical characteristics and the environment, much like a drab and uninteresting puzzle piece may be the only one to fit a particular opening in a puzzle.

We are not wolves or lions...they may have physical competition for leadership roles or the right to mate, we love, venerate, and protect all members of our groups, particularly the elderly. There is significant fossil evidence of this phenomenon, badly wounded members who have healed and returned to health, elderly members with significant disabilities and living with those disabilities.

I suspect the main reason that we age and die is that evolution requires new generations and there are diminishing returns to the group past a certain age. Obviously, we are a VERY long lived species in comparison to most, the Biblical "three score and ten" (70 years) allotted life span goes back around 3000 years and just as obviously we have evolved to live that long. This suggests significant group survival benefits for having older individuals, and humans/hominids have survived through the ages as groups, not individuals. The more interesting question seems to be why we live so much longer than most animals, not why like all animals we die;)