it's easy to get the alcoholic label hung on you as a teenager - just do something stupid while drunk, get caught, and it can easily snowball into "you are an alcoholic and should never drink" and you coerced into AA or rehab or both.
i'm no expert on addiction - it can happen at any age. an easy indicator is when you get older and graduate into responsibilities - marriage / mortgage / job / kids - and you are still partying ( getting drunk on purpose ) into your 30's and 40's - vs. moderation. Also, when you get older ( late 30s /early 40s ) your tolerance drops, and the weekend blowout can easily replaced / complimented by the weeknight buzz.
either way, it's all a slippery slope - i think the high school / college gauntlet of binge drinking poses an acute threat ( alcohol poisoning / dui / death from car crash ) - while the stage of life approaching middle age might represent true physical addiction from 15-20-25 years of drinking.