Why was this old lady so mean to me today?

I was checking out a kids book for my little cousin who is coming tonight so we can read. And as I was checking out an old lady came up to me and said " Geez a teenager checking out a kids book what a surprise! I wonder why today's teens are so stupid. Are you supposed to study for your act or what ever test you are supposed to take? Are you mentally retarded? And why are you so fat for? I wonder why more than half of Americans are fat. People just like you." Nobody defended me at all, nobody never told the old woman to leave me alone or anything they just watched it happen.