How can I describe why smoking is not fun for younger people but also for bit older people too?.
The first step to doing this is to figure out for yourself what makes smoking "fun" for your audience. Then you must try to figure out what sorts of things you could say that could prove that those reasons do not make sense.
For many people, smoking is seen as something that makes them seem grown up, cool, and attractive to the opposite sex. If this is the case where you are, you might want to argue against that idea.
Perhaps you could point out the bad smell that is associated with smoking. You could talk about how a smoker's car and clothes and room and such all smell of stale smoke. You can use the old line about how "kissing a smoker is like licking an ashtray."
What is most important is that you should think about what makes smoking attractive and then try to craft arguments that counter that idea.