How you you spell non self destructive?

This is a matter of style. dailywritingtips.com suggests the best way is:

non-self-destructive

And uses some rather complex reasoning for this decision. On the other hand, in this answer (which I believe is answering a duplicate question), it simply says it's generally best to add another hyphen when a hyphenated prefix is being added to an already hyphenated word. The example in that question is:

semi-self-sustaining

Also, I can't imagine "non" standing alone as a word without being joined either as a prefix to a word without hyphen, as in "noncompliant", or with a hyphen. So there's another line of reasoning to consider.

I concur. Semi-self-sustaining would be correct.

There are actually a lot of "rules" around hyphens. Here is a good source:

https://www.grammarbook.com/punctuation/hyphens.asp

@JasonBassford It would be a non-ice-cream-sandwich or just sandwich perhaps.

It the "12-year-old boy" example. It would depend on context. If you talking about a boy, older than 11 and younger than 13; it would be a "12-year-old boy", or perhaps more correctly, a "twelve-year-old boy".

If you were instead talking to your "old boy" (an English colloquialism), then it could be a "12-year, old boy".

"How old is that Scotch?" "It's a twelve-year, old boy."

Non self-destructive means it’s not destructive to self. Non-self destructive means destructive to a non self—which is nonsensical. Non-self-destructive is punctuationally incorrect in that it suggests both non self-destructive and non-self destructive, leaving it up to the reader to decide which—punctuation marks are used to clarify not to confuse.

The Chicago Manual of Style, sixteenth edition (2010) actually takes three shots at offering a coherent guideline on this point. Two of those attempts are arguably helpful:

6.80 En dashes with compound adjectives. ...

A single word or prefix should be joined to a hyphenated compound by another hyphen rather than by an en dash; if the result is awkward, reword.

non-English-speaking peoples

a two-thirds-full cup (or, better, a cup that is two-thirds full)

...

7.85 Hyphenation guide for compounds and words formed with prefixes. ...

non nonviolent, nonevent, nonnegotiable, but non-beer-drinking

What these two guidelines suggest is that if you would hyphenate self-destructive (and I certainly would), you should attach the prefix non to the compound term with another hyphen.

Bryan Garner, Garner's Modern American Usage, second edition (2003) adopts a similar analysis in discussing phrasal adjectives that begin or end in a compound noun (not entirely on point here, but useful as an illustration of a similar situation:

Phrasal Adjectives...

E. The Compound Conundrum. When the first or last element in a phrasal adjective is part of a compound noun, it to needs to be hyphenated: post-cold-war norms, not post-cold war norms. Otherwise, as in that example, cold appears more closely related to post than to war.

For some reason, Garner doesn't consider the possibility of rendering the phrase as "post cold-war norms" to be worth discussing at all.

The U.S. Government Printing Office, A Manual of Style (1986) reaches the same conclusion as Chicago and Garner, but it takes a different route to gt there:

6.32. Use a hyphen or hyphens to prevent mispronunciation, to insure a definite accent on each element of the compound, or to avoid ambiguity.

[Relevant examples:] non-civil-service position, non-tumor-bearing tissue

The only discordant advice I came across was from Words into Type, third edition (1974), which offers this unusual prescription in situations involving "Compounds with noun plus '-d' or '-ed'" (again, a slightly different case from the one that the poster here presents):

If the first part of the compound is qualified by a preceding adverb, omit the hyphen.

[Examples:] fine-grained sugar [but] extra fine grained sugar

I don't understand the thinking behind Word into Type's guideline, but if I were looking for a justification for rendering "non-self-destructive" as "non self destructive," WiT's coverage is about as close as I could get to an analogous form in a published style guide.

Overall, however, the style guides I consulted seem much more amenable to the form "non-self-destructive."

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Reviewing the logic of each of the alternatives that the poster suggested, I offer the following observations:

Non self-destructive: The problem with this option is that it treats non as a standalone word, rather than as a prefix. But if you use the form non-destructive (or nondestructive) to express the idea "not destructive," you are using non- as a prefix; and it's hard to see why that term should suddenly transmute from a prefix (non-) into a freestanding word (non) just because you are trying to express the idea "not self-destructive" instead of the idea "not destructive."

Non-self destructive: The problem with this formulation is that it sounds more concerned with the "destruction of the non-self" than with the "non-destruction of the self."

Non-self-destructive: This is the form I would use, for the reasons discussed earlier in connection with the style guide discussions.