This is a very good question. I also did not start drinking habitually until I was around 25. I also struggled with depression through my teenage years into early 20's but even though I had drank before I did not use it as a solution for my depression. I just had good days and bad and I didn't put much thought into it.
When I began drinking as a habit it was only a few drinks on the weekend or at social events I would go to with my husband or friends. As time went on it became part of my routine and something I really looked forward to. I remember thinking that anyone would be crazy not to drink to relax.
Over time my tolerance grew and I drank more often. I certainly didn't know at the time that my brain was starting to change and my natural ability to balance my brain was regressing as alcohol took over.
I guess that's why it is so hard to get back that natural balance once we quit because it takes years to become fully addicted.