Ageism works in both directions. As a teenager in the public eye people would talk condescendingly to me. When you get older there's this feeling that you have to start carving up your face and body. Right now I'm in the middle ground - I think women in their thirties are taken seriously.
Just as we reject racism sexism ageism and heterosexism we reject speciesism. The species of a sentient being is no more reason to deny the protection of this basic right than race sex age or sexual orientation is a reason to deny membership in the human moral community to other humans.