I would tear the pack out of the teenager’s hands, smoke every cigarette in it to the nub, and lick the packet clean while heaving with delight.

Which would probably put the teenager off the weed but do nothing positive for my well-being.

Sorry, I’m not in a good position to try to prevent anybody else from smoking. I’m barely managing to prevent myself.

I’d say “it” to refer to a baby but for anybody older you can’t use “it”. If it’s an imaginary person or the sex is not known, you can say “they”. Sometimes, though, that’s confusing because people might think you are talking about a group. In these cases, I randomly use “he” or “she”.