You should try to act as the law abiding citizen and none of your activities should be against the law. Keeping in mind the serious repercussions of breaking the law, you should never buy tobacco for the underage people.
If any teenager asks you a favour of providing tobacco to him, tell him that it is illegal and neither of you should do that. Try to explain him in a gentle way and emphasise the need to follow the law.
If you have ever facilitated a teenager by providing him tobacco, you should contact him and tell him straight away that you will not do that in future. You should tell him that it is illegal and he should not seek help from anyone otherwise they can be held liable and this can have a deep impact on his future.
He might try to convince you by opting various ways. You should not go soft on him and do not fall for his words.
If he still insists you for a favour, remember that he is not a good friend to hang out with as he might be just using you.