The best room available, the only decision possible, the worst choice imaginable -- as it turns out, postpositive* adjectives happen, even in English.
The phrases above would still make sense with a more conventional word order: the best available room, the only possible decision, the worst imaginable choice. There are, however, other phrases which only make sense when the modifier is postpositive: the best room available in the city, the only decision possible at this time, the worst choice imaginable under these circumstances.
The Wikipedia page concerning the postpositive adjective includes an interesting example:
I'm here to find the responsible people.
I'm here to find the people responsible.
We tend to read the first as meaning people who generally do the right thing. We tend to read the second as meaning people who have done something wrong. This is a very large change in meaning that results from a very small change in position.
I'm here to find the people responsible for this mistake.
Just like "available in the city", "possible at this time" and "imaginable under these circumstances", "responsible for this mistake" is naturally postpositive. The word "responsible" does not modify "people" on its own. It is part of a larger phrase that as a whole modifies the noun.
When we encounter "the people responsible", we assume that it's the people responsible for something specific -- people who are circumstantially responsible. When we encounter "the responsible people", we assume that it's people who are inherently responsible. This explains why "the responsible people" seems good while "the people responsible" seems bad.
after 20 sober years
after 20 years sober
A similar but more subtle shift in meaning occurs here. "20 sober years" can easily be read as 20 serious years, 20 grave years, 20 dull years, 20 boring years -- the years themselves are inherently, intrinsically sober, and only those senses of the word "sober" that can directly apply to the years make sense. In the original, it is not the years themselves that were sober. The man was sober. He spent those 20 years refusing and avoiding intoxication.
"After 20 years sober" expresses the same general sentiment as "after staying sober for 20 years" or "after 20 years of sobriety**".
* Why "postpositive" instead of the more sensible "postpositional"? "Postpositional" would naturally pair with "prepositional" -- but English grammar already uses "prepositional" and "preposition" to mean something entirely different. The oddity is that a so-called prepositional phrase is ordinarily postpositive. It would be far too confusing to discuss the common postpositional prepositional phrase, so we won't.
** Because the word "sobriety" exists, there is no need to form "soberness" out of "sober". If "sobriety" didn't exist, "soberness" would be a reasonable formation.