Even if he didn't, there's a lot of evidence that Johnny Boy was exceptionally bright, to say the least. You can't really expect that level of writing skill from a typical 10 year old.
For what it's worth, I believe a reasonably intelligent child could have written that. Here's another letter, this one written by a 12-year old girl in 1842, which is similar to the other except for being, if anything, a bit more composed.
https://100yearsofstories.wordpress.com/2016/01/06/my-deares...
> My dear Papa,
> It is with much pleasure I write to you these few lines to inform you that our vacation will commence on the 18th of the month when I hope you will find me improved in all my studies in which I have done my best. Miss Sykes and Miss Martha present their compliments and hope though late you will accept their thanks for the very nice hare you were so kind as to send.
> With love to all at home, you remain my dear papa.
Can't you? I've taken the quote from the parent comment and replaced every punctuation mark and coordinating conjunction that separates independent clauses with an interpunct, and bracketed the relative clauses.
> I love to receive letters very well • much better than [I love to write them] • I make but a poor figure at composition • my head is too fickle • my thoughts are running after birds eggs play and trifles [till I get vexed with myself] • Mamma has a troublesome task [to keep me steady] • I own [I am ashamed of myself] • [If I can but keep my resolution] I will write again at the end of the week • give a better account of myself • I wish you would [give me some instructions with regard to my time] • advise me how [to proportion my Studies and my Play] in writing • I will keep them by me • endeavor to follow them • I am with a present determination of growing better • [if you will be so good as [to favor me with a Blank Book]] I will transcribe the most remarkable occurances [I meet with in my reading] [which will serve to [fix them upon my mind]]
As you can see, there is no nesting deeper than one level except at the very end. There's a range of vocabulary, but most of the words were common in English at the time of writing and have simply fallen out of fashion. 'Fickle' is usually now 'impatient'; neither expresses a more complex idea than the other. Assuming the letter is representative of genuine sentiment, I find his desire for personal development more impressive than his language; indeed his commitment to reply to correspondence promptly is the most positive reflection of his character.
English styles changed. For example, Hemingway was brutally brief.
I believe (but don't know) that the further from the Anglosphere one goes today the longer and more ornate the written language tends. Anyone know how to confirm or deny? I have only anecdata from those who were high-school educated in Farsi or Russian before moving to the US.