Not that I entered this discussion intending to go "deep", but here we go :P
I started drinking properly when I was 15, and smoking when I was 17. Luckily (I guess), I always do them together - I never smoke when sober, as it makes me physically ill. That said, I do like going out and drinking, having a "night out", clubbing etc. which often involves smoking. My rational is that, if there's any time in my life to prioritise having fun over health/safety, it's between 18 and 24, so I'd rather enjoy my life tomorrow, than regret not doing so in 60 years