As a teenager, I have had a more recent experience of being a kid...
In my culture, children must not take part in adults discussions, as it shows them as acting too grown up at their age, like a prematurely-ripened fruit. In fact, I tried to go against this rule, but over time I realised that the adults listened to me a lot more when I talked less, and spoke only important things to them.
As for your question on how to tell if it's really you who's acting childish, your close friends are generally the best judge, as they know you pretty well. But as a rule of thumb, always put your hands behind your back, and speak at an audible volume that's not too loud, but not too soft either, remaining polite at all times. Keep in mind that adults generally trust no one's decision except their own, and knowing that, be persuasive, not aggressive in your tone.
And, when you are a hundred and ten percent sure that it really isn't you acting funny, but an adult is still not taking you seriously, tell them what you came to say once again, with a slight firmness, and see if they're convinced. If they're still not taking you seriously, and the matter is truly serious, possibly take it to a higher authority.
I've tried to answer in a way which would help 'kids' in general, it would still be important to know where you are from, as people from the respective place could give you better insight.
