This presumption is almost always incorrect but what many people don't realize is that there is a category of books that are not very old but can be worth a lot of money because, despite being relatively recent, they are quite rare and considered to be important cultural milestones.
Modern firsts is one of the most active areas of book collecting today for a number of reasons.
Because the books are modern, they are often available in sufficient quantities, even as first editions, to be able to satisfy a large body of collectors. Modern firsts can be broken out into a handful of different sub-categories. There are those writers from the latter half of the century who established themselves as major literary figures, but whose works have not yet stood the test of time as much as their literary forebears' works have. Norman Mailer , Philip Roth , Ken Kesey , Raymond Carver fall into this category. There is a speculative edge to the modern first editions market in which these writers' works are actively sought and traded.