Hey OP, I can totally relate to you. I was a straightedge guy all through high school and college, never drank a drop of alcohol. Even when I turned 21, at I believe 23/24 I had my first beer (this is after college) and some liquor.
It wasn't until I was 26 that I started drinking heavy. It went from a glass of liquor to occasionally to cut nighttime stress/help me sleep, to 4 or 5 glasses of wine EVERY night (sometimes more).
This came off the heels of a very bad breakup I had back in 2012 and has lead to where I am today.