Can you tell me about Jiu Jitsu?

Everyone is telling me that I am too soft and my mother made a statement to me saying that I cannot fight with anyone. I am sick of it, so I am decided to take up some Jiu Jitsu( I have just turned 28 today). Does anyone here practice this martial art? Can it make you loose weight? Is it effective against multiple attackers? Is it easy to take up? Am I too old for it?