

As bitjunkie said, if you were referring to 90s in the possessive, it would be 90s’ since it’s a plural noun already - much like parents’ mortgages or stores’ buildings.
That said, I would probably look at the phrase “90s child” as either (1) a compound noun not needing anything to be possessive (like “ice cream”), or (2) with “90s” as an adjective modifying “child” (like “latchkey kid”).
Modem starts dialing up. Everything is going fine, attempting 28.8k negotiation… failed. Okay, now I’m listening to 19.2k negotiation… failed again? Ugh, ok, 14.4k… no? Jesus fuck. Please let me at least have 9600…
Edit: typoes