Well, if we count the first pet that was fully my responsibility, that was my 3 year old cat Nell, and I got her when I was 17 (I just gave away my age here ). But there has been a sad history of pets that have gone through my family, some of which were "mine". The first one I remember was a rabbit at my fathers house, white with red eyes. I have only seen him for a couple of times, and he wasn't kept in good conditions. I have no idea what happened to him.
The next one was a cute little hamster. I got him with a cage with the wheel and everything. I was about 8-9 years old at the time, and I knew nothing about hamsters and how to take care of them, and neither did any of my family (nor did they bother to learn). He wasn't very active, but he wasn't unfriendly. My mother could hold him in his arms and he wouldn't object, but I was too scared to do the same. He met his end when someone left his cage on the balcony with the door opened, and he ran through the hole (we lived on the 4th floor).
After that there was a turtle, who was kept in a round flower vase, partially filled with water that was never changed. We had her for a week or two, and then she was given away (I hope). There was a cat and a rabbit after that, the cat was quickly returned to the streets, and the rabbit was given away to a person who kept a rabbit farm. The cat was meant to cheer me up when I was going through some tough times, but the whole situation just made me more depressed. The motivation for getting the rabbit will probably be a mystery to me forever.