I didn't have my first blackout until 35. I started abusing alcohol a few years before that. Up until that time I didn't like the feeling of being drunk and would actually throw up if I tried to drink more than 3 drinks in a sitting. I adapted through regular heavy drinking to the point were I ended up with no control.
It doesn't seem like it is that unusual either. There have been other women (probably men too) that have said the same thing. People get into the habit of having a few at night, then tolerance sets in, which changes our bodies and minds.
I didn't drink when I was young because I was isolated. Even after I moved out of the house I was too busy or just not interested in it much. I only drank when I went out to the bars and usually I would be the driver because I didn't like to drink too much. Then I got married and had a baby. I worked nights and barely had time to sleep, let alone drink. Then I moved to working afternoons and started having a few drinks after work each night to hang out with friends. A few years later I had my first blackout and I knew something had changed but I kept on drinking for another 10 years.