I also haven't any children so I'm going by my own parents views & to some degree my own!
Teenager , on the grounds that any younger it's a safety issue.
Everyone starts out saying never, but in reality it's a different story! So I can't answer this one.
2-3yrs because toddlers want to see what's going on around them.
Never, once again on safety grounds.
Parents should never smoke but if they do they should do it outside.
Difficult this one, you have to trust when they are Teenagers but we all have been Teenagers!.
I would never give a child Alcohol in any circumstances