Since 12 if memory serves right. I'm 42 now - so that is 30 years straight.
Always wanted it. I remember I resolved to buy a poison at age of 12 (because many characters use poisons in adventure books) and one sly foxy guy told me that he has got one. He said, however: "Dont drink it, you need to inhale it, and you'll pass out". I traded it for toy cars or soldiers, but it appeared to be some medicinal liquid, formaldehyde I think. He scammed me :)
I'm positive that it's my nature, if it has been with me for all of my life. I don't mind.