I smoked my first cigarette in about the fifth or sixth grade. I was so disgusted by that experience that I didn't smoke another cigarette again...until my first weekend in college. And, it wasn't long until, well, you know...
I don't think any of us set out to be addicts but that occasional social cigarette early on can become an addiction quickly.
I think it is good that your daughter told you and I really hope she doesn't light up again. Since she confessed to you, I hope she listens to what you say and realizes how bad this whole smoking thing can really be and steers clear of trying it again in the future.