It also depends on how active your lifestyle is.I wouldn't suggest a kitten or a cat because you have to be the cat food,which can cost you tens of hundreds dollars a year depending on the cat.You also would have to train it use a litter box or if you choose to make it a cat that does its business outside,you would have to clean it up.I also wouldn't suggest a dog because unless it's an older trained dog you may not want it around children that can't control him/her.What I would recommend is a fish,a Koi can live for almost 15 years at least if you take proper care of them.Betta Fish are also good choices.They come in a variation of colors and are easy and cheap to take care of.Another good animal is a turtle.They are cheap and easy to care for,but can be very loyal animal. Some turtles can live for 100 years.Like I said before they are cheap and easy to care for and that is perfect for a 12 year old.Finally Turtles and fish to my knowledge do not affect any allergies.
I agree with AdR1aN15, it really depends on how responsible you are. If you are a lazy kid who just wants to have a pet, get a fish :)) But if you are an active one and you really want a friend, get a boxer (dog). They are most playful, smart and lovable dogs! You will never regret if you will have a boxer :)
It depends how much care you are willing to put toward the pet. I got my first guinea pig when I was 5, got my first dog when I was 6, got my first hamster when I was 8 and got my first rabbit when I was 9. The easiest was the hamster, but they don't live long. Just get a dog, they are loyal and a good friend.