The good kids study the
Official Wiki (the bad kids study the Fandom Wiki)
From the study of the Official Wiki It is now ascertained that you will NOT, ever, lose an expansion because you did not unlock it in the age it is attributed to; the saying "an expansion is attributed to an age" only means that that expansion becomes availabe upon entry into that age and it does not imply that that expansion expires by the end of that age (i.e., entry into the next age). No, it does not expire. It remains available until end of game or until when you place it, whichever is earlier.
P.S. How's my explanation? Does it now sound much more professional than my "Deepity Thoughts" thread?