Some ways to interpret this plot:

If you look from y-axis, the more gray you can see for a particular age, the more value it is to the society. That way, a 1-year old is of the most value to the society and a 25-year old is of the least value to the society.

As we saw for animals, a youngling is very valued for humans as well. We see that 1-year old is the most valued and that infinite value starts stopping around 18-year old. After that, an older person is more valued than their younger counterparts.