I don't remember when I had the very first alcoholic drink in my life... it was probably a sip or two, or a glass of champagne when my parents drank (very rarely, I did not have drinkers around me in childhood). I do recall very vividly the first time when I got fabulously drunk on liquor - I was ~13. After that, I had a few years of intense and crazy partying with lots of binges on weekends mainly until ~17, when I quit the whole scene because I got bored with it and discovered other, more "mature" interests (mostly studying). I drank very infrequently from that time through most of my 20's. When I did, it tended to be quite excessive, though.
My real problem drinking (alone at home) started in my early 30's during a very stressful work-related period that more or less put me into the habit of everyday or near-everyday drinking. I quit this year one month before my 40th birthday.
So ~8-10 years of clear alcoholic drinking habit in my 30's, but I did drink on occasion starting long before.