Growing up, I considered coffee an adult drink and enjoyed it black or as a cappuccino (and still do).
Then beer and liquor were considered adult drinks and I infused my own alcohol while enjoying craft beer and got into bourbons.
As I get older (and I'm just pushing 40), I'm realizing that water is the adult beverage. I still imbibe and enjoy it, but water makes me feel healthier when drinking it.
Then again, I run on nicotine and caffeine (and sometimes hatred ... just kidding, well, maybe) so water tends to replenish what I abuse throughout the day.