@Skeeter Lewis - "some descriptivist dictionaries" - Could you perhaps name a dictionary that isn't descriptivist.The OED, for example, never set out to be anything else. Even the American Heritage Dictionary, which set up in opposition to the allegedly free-and-easy Webster's Third edition, is basically descriptive, with its user panels.

Incidentally, the group of soldiers meaning seems to have predated the tenth of a legion meaning (Online Etymology).

The overwhelming use of cohort today has nothing to do either with the military or with sidekicks, but with statistical analysis and social science. Out of 360 instances of cohort at the British National Corpus, 97 are 'a cohort of' (only one of which appears to have a military connection), 11 are 'age cohort', 29 'birth cohort' and 29 are 'cohort study':

http://www.just-the-word.com/show_examples.pl?triple=cohort_N+pmod+of_PREP

See also examples at the Economist:

http://www.google.com/search?hl=en&q=cohort%20site:www.economist.com

The use you don't like seems to have started in the States in the 1940s/50s, including several outings in the linguistically rather fussy New Yorker, one being by JD Salinger (who seems to have had a bit of stick for starting the fashion). The Times Literary Supplement of Nov 1965 is on your side, calling it "the new American vulgarism of 'cohort' meaning 'partner' " (from MWDEU). But it seems pretty commonplace nowadays, and in a Usage Note at the (nominally conservative) American Heritage Dictionary they say:

"Seventy-one percent of the Usage Panel accepts the sentence 'The cashiered dictator and his cohorts have all written their memoirs', while only 43 percent accepts 'The gangster walked into the room surrounded by his cohort'."

You can read about 'cohort' in the Merriam-Webster New Book of Word Histories at Google Books: http://books.google.pl/books?id=IrcZEZ1bOJsC&pg=PA114

and in The American Heritage Book of English Usage, also at Google Books:
http://books.google.pl/books?id=BEHFyMCdwssC&pg=PA81

I can't help thinking that cohort is in a class with decimated.

@dlockner - I'm afraid wrong on both counts. A malapropism is neither intentional nor to do with spelling, but rather the unintentional use of a similar-sounding word instead of the one you meant, 'often with a comic effect' (Oxford Dictionaries Online), for example 'dance the flamingo' instead of 'dance the flamenco' (although that wouldn't actually be so bad, seeing flamenco is Spanish for flamingo).

re: cohort. This is from Oxford Dictionaries Online:

They give three meanings: the first is the one given by Skeeter Lewis.

cohort - 2.[treated as singular or plural] a group of people with a shared characteristic or a common statistical characteristic:
"a cohort of civil servants patiently drafting legislation"
"the 1940-4 birth cohort of women"

cohort - 3. often derogatory - a supporter or companion:
"young Jack arrived with three of his cohorts"
"a long-time cohort of the band"

@jayles- they probably got it from the OED (1558) - I've got 'upon the sudden' from 1585. I think you're much better looking at Google Books than Google as you can narrow down the dates and miss out all the dross.

Anyway I'm putting it all into a post, and will give details when finished. One thing is clear - 'on a sudden' carried on co-existing with 'all of a sudden' during the eihgteenth century.

OK I've found the 1558 one, but you can't get in to check

http://www.google.pl/search?q=%22upon+the+sudden%22&hl=en&sa=X&ei=OrYEU7m2Cer14QTakYDgAw&ved=0CB8QpwUoBA&source=lnt&tbs=cdr%3A1%2Ccd_min%3A1500%2Ccd_max%3A1600&tbm=bks#hl=en&q=%22upon+the+suddeyne%22&tbm=bks&tbs=cdr:1%2Ccd_min:1500%2Ccd_max:1600

Shakespeare - sudden used as a noun:
on the sudden, upon the sudden - 8 instances
on a sudden, upon a sudden - 4 instances
of a sudden - 2 instances
none with all

It looks as though the 'the' version came first.

@jayles - and from your other link (phrasefinder) - " 'All of a sudden' sounds like the kind of poetic version of 'suddenly' that would do justice to Shakespeare. In fact, that's what Shakespeare thought too, as it was he who coined the phrase. In The Taming of the Shrew, circa 1596, we find: Is it possible That love should of a sodaine take such hold?"

So Shakespeare coined the phrase? Around 1596?

"Behold of a sodaine behinde me, I heard a rusling noyse, like the winde or beating of a Dragons winges" - Hypnerotomachia, the strife of love in a dreame - Francesco Colonna, ‎Robert Dallington - 1592 (no apostrophes in those days)

http://www.google.pl/search?q=%22of+a+sodaine%22&hl=en&biw=1018&bih=595&sa=X&ei=ZbADU-CpK6jw4QTcvoDwDg&ved=0CB4QpwUoBA&source=lnt&tbs=cdr%3A1%2Ccd_min%3A1%2F1%2F1500%2Ccd_max%3A12%2F31%2F1600&tbm=bks

@jayles - had a far healthier life, no doubt. From your Grammarphobia link -' “All of a sudden” first appeared in 1681' - now there's a challenge for us Googlers:

"therefore all of a sudden they came with great violence unto Aaron, urging him to make them one" - Seven Godlie and Frvitfvll Sermons, 1614

"for Antichrists apostacy was not at the highest all of a sudden, but encreased by flow (slow) paces" - David Pareus, 1644

"He tells him what a disgrace it would be for him, if without order of law he should ( all of a sudden) bring so man Noblemen to the scaffold without a crime" - The History of the Houses of Douglas and Angus - David Hume, 1644

Now I can answer your question - relied on the OED to give us earliest appearances. But nowadays, if it's into the printing era, we can often beat the OED, just by looking in Google Books.

http://www.google.pl/search?q=%22all+of+a+sudden%22&hl=en&sa=X&ei=-qkDU8fxD8Wx4wT1gIHIAw&ved=0CCAQpwUoBA&source=lnt&tbs=cdr%3A1%2Ccd_min%3A1%2F1%2F1500%2Ccd_max%3A12%2F31%2F1650&tbm=bks

@Emera - cannot, can not, can't are all use for both "not allowed to" and "not able to"

You cannot / can not / can't smoke in here
She cannot / can not / can't come tomorrow

Oxford Advanced Learner's Dictionary (and Google Define) - "cannot = can not"

The main difference is that cannot is much more common than can not. The only exception is in the type of expression discussed above - 'You can NOT go if you don't want to go' - here cannot wouldn't work.- i.e. it is possible not to go.

http://www.macmillandictionary.com/dictionary/british/cannot

@jayles - I should have put a smiley after 'nice try'. I didn't mean it in a negative way.

@Matt K - rhetorical questions are still questions:

What have the Romans ever done for us? - Life of Brian
Is the Pope a Catholic?
Smoking can lead to lung cancer. Who knew?!

All taken from Wikipedia's entry on rhetorical questions.

And that upside down question mark is for introducing questions in Spanish:

¿Cómo llegas a tan tarde? - Why are you so late?