To address your actual question about how to deal with this is another issue altogether. These days people just get away with whatever they want, it seems.
Are you going to turn away a guest upon arrival because they bring up where they can go to smoke? Very awkward.
For guests that lie about their age - I have this issue, too. We have a 25+ age policy (others in my area have raised that to 30+). But it seems the younger they are the more of a trail they leave. It’s amazing how much you can find online. If I find that someone is lying about their age I sometimes will ask them for an image of their drivers’ license or other proof of age.