What is the best age to introduce your kids to pet? I want to teach him reponsibillity by having one.

Since your child has to be responsible for his pet, I would advise that he be more than able to take care of his daily needs (feeding, bathing, changing). A safe age would be around 8 or 10 years old. You will have to be prepared to look after the pet if he happens to lose interest after a while, as a pet is not a toy, but a lifetime companion. Perhaps you could borrow a friend's pet to see how well he adjusts to having an animal in the house.