young, is it a problem?

I always thought being young is something you'd feel proud of or you can brag about. Nothing falls down as a reason why I thought so. I can't think any of 'em. But, somewhat I had kept thinking so, until now. Now that I got job at English private institution, being young is something you have to avoid to let others know by any means. The kids there can think you easy or something so treat you really bad. They asked me how old I am. So, I just gave them right answer. And, the head teacher 'princess' was like, "you are not supposed to tell them your real age. It'd be better to just tell them you are 25 years old or something" I know she is right. She has more experience on this case. But, I still felt very embarrased. I felt helpless about it, about my age, being young as I am. Okay, gosh, I want to be 25 years old right now if I can.
So far, I had a chance to teach 4 classes. Well, how was it? It was hard. Not just hard, but really takes lots of energy out of me. First class was for young kids like 8 to 9 years old girls and boys. It wasn't that hard. They didn't really understand my English when I spoke in uncontrollably fast speed. But, they listened to me and paid attention on the class not like the Second class kids. Students in the second class were composed of 10 years and 11 years old boys and girls. They were fucking brats. They didn't listen to me once, just did whatever they wanted to do. They were rude as well. It really pissed me off when they spoke up and all screamed at the same time. One girl even cried at the last minute of class because she lost the game so that couldn't get the sticker which is equivalent to money. I don't even think about them anymore. They were the best at being the devil!!! The third class was definitely better than other classes I took before it. They were mostly in the last year of elementary school, so it was easy to teach them, except one girl was really crazy keeping the whole class distracted by her unnecessary and weird behaviors. The last class was TEPS class. So, they were the top student in the academy. It helped me because they REALLY listened to me. However, the problem this time was not them, but me. I couldn't teach them really well due to my English or Korean skill. Argh. I hate it! Like, I really know what those all mean in English, but I can't help not knowing equivalent Korean words for them. I struggled a lot for this class, and I guess it won't let me go away from struggles unless I don't teach this class anymore.
The experience in English academy in Korea doesn't end in here because I have 2 more months to fully experience. I'm quite scared. But, who knows? I can maybe do much better or I can be fired today or tomorrow...sooner or later. But, this can surely be really a good experience for me, at least I hope!