I agree with Ron and the others, but here's what I'd do...
Go to your dad (or Mom I suppose, I guess I'm being sexist, lol) and say that now that you're a man you want to make some of your own decisions, and although you could have simply snuck off and smoked a stogie without telling him, that you chose to come tell him that's what you'd like to do.
If my son did that, I'd gain a lot of respect for him for acting so much like an adult. I think your dad will feel honored that you talked with him first, and in all likelihood he'll tell you that you can make your own choice about it (though I don't know your dad, so I can't tell you what he'd say).
That's what I would do, and what I hope my son would (will?) do. But if you go behind their backs to smoke cigars, they'll start thinking that you're doing much worse things behind their backs as well...epic fail, lol. Being 18 and living at home is HARD ENOUGH without adding more drama.
So if you think you're man enough to smoke a stogie, then you're man enough to talk to your dad about it first.