If you've ever spoken at a school in front of a group of high school girls and boys, you'll realise how impressionable they are. If you know what type of information that would interest them (i.e. if you don't understand social media, forget it- they'll think you're an alien) and can intertwine your own message for them in an engaging way, they will remember one-liners and facts you mention for an entire lifetime. They may very well just make decision after decision based on that one thing that stuck with them for whatever reason. I've seen and experienced this time and time again, I know this was the case for me and look what my life's work has become....
So, this means if you're speaking with a group of high school teenagers or even one-on-one whether they be a friend, a younger brother or sister or one of your own:
a) You want to make damn sure you've given them accurate information
b) You'll realise how easily teenagers can be influenced.
Let's address a) first.
If you don't know something for sure, DO NOT WIKIPEDIA/GOOGLE it PLEASE. If you were getting life saving surgery would you not get an expert opinion?! Of course you would. That means, when it comes to the health habits we have EVERY SINGLE DAY the passing comment you weren't actually sure about that you gave the open-minded, willing-to-try-anything-to-fit-in teenager could do a lot of harm. They're at an age where they don't talk to their parents most of the time, and rely on each other (other impressionable teens) for advice. The advice they take on board and execute might become a daily habit, and you would be none the wiser. Do your research or simply be honest if you don't know and while it's easier to trust the net than educated health practitioners these days at least get a few opinions from the experts to compare it to. To compare advice from a Nutritionist, Naturopath and Medical Doctor is MUCH more effective than comparing it to Wikipedia, Google search results and a journalist's article (who has studied Journalism NOT Health).
I realise this sounds quite commonsensical (but you'd be surprised) and sometimes we need a reminder and that's OK.
Look at the evidence. Teenagers are easily led to believe that what their celebrity favourites are doing, consuming, wearing is the result of an organic appreciation and results, and NOT paid marketing fees that end up straight in their pocket to endorse said products/activities. Those of us with a bit more life experience know better (and you can try to explain this to a teenager but it's not a debate you'd usually win), and in fact realise 99% of what teens are buying is based on the deception of a good marketing campaign with a good script for the celebrity to deliver to camera and a big cheque for their morals to be somewhat compromised in order to be an ambassador for it. So I think it's safe to assume they are gullible until they learn life lessons the hard way.
Please don't let their health be one of them because there's no coming back from terminal illness or a lifetime of poor health choices. Let them dye their hair horrid colours, or buy stupidly expensive tech gadgets. Just please don't let them have coke or the whole block of chocolate, or too many drinks while they're underage IF you can help it. Don't give in to them if you have any patience left because they will associate winning with the reward of poor health choices, and will associate being fed up with enabling poor health choices to be made, just as you did - it goes both ways.
They will role model YOU. They'll watch you and want to emulate you, and the best gift you can give them in return for the flattery is to show them it's COOL to be healthy. They'll do it at home, at school, at work. Then all their friends will do it too. Then they will age... and they will still be doing many of the things they've always done because humans are creatures of habit.
The less poor choices they make --> the less damage they do to their bodies --> the less money they spend in their lifetime bouncing from doctor to specialist when it the damage is already done --> the less stress and strife as an adult with responsibilities --> the less pressure on the national healthcare system --> the more focus we can have on preventative medicine --> the more well-rounded, educated, confident, healthy individuals of the next generation --> the more productivity they generate for themselves and their businesses --> the more positive change for the planet...
You can't blame people for what they don't know they don't know. So let's educate them, so they have no excuse.