human beings treat other humans according to their 'survival and replication' value. that is, their ability to survive, and their ability to have offspring. older generations are not respected in society, because they are infertile and prone to health problems. similarly, society does not respect fat people because they have a low s & r value.
that's also why we act nicer to good looking people than to ugly people. good looking people are judged to be in better health, so they are deemed more valuable.