That is correct, all you have to do is own an opinion. You could be wrong. You could be correct. How can you prove it ? There is not enough evidence in the letter. If your opinion were strong enough you could go to extraordinary lengths, even publishing your personals to make your claim to authority the basis of your argument but I would not recommend that at all.
No, I do not call myself an educator although I am called upon to educate others.
No of course I can't "prove" this is a kid or an adult. Can you "prove" there is a GOD? I can't, but I have absolute faith that a dead man got up out of a grave 2004 years ago next Sunday and said that I can too.
My opinion that this was not written by a kid is based on my vaild experience as a teacher for some 15 years (the last seven in an inner city high school). Some pigs are more equal than other pigs if you remember your Orwell. My point is that my experience and therefore my opinion as an educator trumps others if they're experience is in a different area. The opinion of a professional writer is just as valid as mine because they are tangentally connected. BTW, I write for publication as well, and I have a professional literary agent.