First off, the scenario as described is hilariously vague. It's so vague (man attacks drunk teen with knife), that it may as well be Jason from Evilroy's earlier example (which...fits, honestly. Dumb, drunk teens are his MO). In which case, obviously you are allowed to protect yourself from the knife wielding maniac, duh. The problem is that, with this scenario at least, is the fact that there is usually a lot that happens that leads up to 'wild stabbing motions' (usually). Context matters, like who initiated the conflict, where did it happen, did any party have friends with them, were there witnesses, etc.
For instance: say an argument at a bar between our drunk teen and someone who he has just bumped into has devolved into a brawl. The adult feels he is right, but unfortunately for him, our drunk teen is also the starting linebacker on his high school football team and built like a brick shithouse, and it's not looking good for him as our teen is a mean drunk and keeps on swinging. Fearing for his life, he reaches into his pocket and pulls out his knife and...you get the point. And this is just a few of any number of ways the vague 'man attacks drunk teen with knife' headline can go, including just straight up self defense for the teen, but then remember I did say;