I was about 10. My grandparents had made wine from quince for the adults and juice for the kids. It was really good but I didn't notice the bottles had swapped and drunk a whole cup of wine.
After that, I think the first time I was drunk outside my parents' control was ~16, severe trouble with abusive stepfather, I went outside school for lunch and went to the bar to drink and play pool. I came back to school drunk several times, around 17 I got expelled from school for drinking too much and passing out on a school trip. I got a note that "your son has a drinking problem", the parents were sure a summer of pioneering would resolve that, during that time, I went to birthday parties of people I met on IRC and various clubs and raves. Until ~20yo I was a heavy drinker, that got better after my first divorce, but continued until I left the JW.