Neither Steve nor Jay is 100% correct, but Jay is closer. I don't have time for a full post on this subject (which I've been meaning to write one of these days), but here are the essentials.
As several people have noted, the modern concept of a "rookie card" did not exist in the early hobby. As I documented in a previous post (here:
http://net54baseball.com/showthread.php?t=233772), it was not until the mid-1960s that dealers even began consistently charging a premium for cards of star players, let alone "rookie cards". When the term "rookie card" was used in the hobby in the 1960s and 1970s, it referred to those multi-player "Rookie Stars" cards that Topps put out every year during that time. During this period, "sophisticated" collectors took pride in not caring who was pictured on a card, only about how rare it was and whether they needed it for their set.
(By the way, Steve is correct to say that before 1980 most baseball cards were bought by kids -- and that remained true for quite a while after 1980 -- but we're talking about the organized hobby that had existed since the 1930s. Whatever kids were doing, the adults who collected baseball cards in the 1950s, 60s, and 70s did not care in the slightest about owning the first card of a given player, as opposed to any other card of that player. Also, Steve is incorrect to say that the monthly Beckett guide began in 1980 -- the first issue was not until September 1984, by which time the rookie card craze was in full swing.)
The first time people in the hobby began caring about a player's first card came when Hank Aaron approached and then broke Babe Ruth's career home run record in 1973-74, and for a few years after that. Aaron's 1954 Topps card began commanding a significant premium on the open market, and a lot of old-time collectors were not very happy about it. When Jim Beckett distributed his first card price survey in late 1976, he asked about the price of only one non-rarity star player card -- the 1954 Topps Aaron. When Beckett presented the results of the survey in the March 31, 1977 Sports Collectors Digest, he called it "Aaron's rookie-year card #128" (see footnote 1 on page 50 below), and discussed the controversy over its pricing on the following page. (My full post about Beckett's first price surveys is at
http://net54baseball.com/showthread.php?t=216495)
By this time (the mid-to-late 70s), prices of star player cards had begun to rise steadily, and cards from early in the careers of superstars were starting to command the biggest premiums. See my post of Lew Lipset's report on card auction prices in 1977-78, including a June 1978 column focusing on the 1952 Topps Mantle. I don't think the words "rookie card" appear anywhere in these columns:
http://net54baseball.com/showthread.php?t=243152
See also these articles from Baseball Hobby News in 1979 about the state of the hobby, including rising prices. I don't think the words "rookie card" appear in these articles either, but editor Frank Barning did discuss the top young players to invest in, which would become a key element of the rookie card craze in the following decade.
http://net54baseball.com/showthread.php?t=241548
http://net54baseball.com/showthread.php?t=241741
In 1980-81, the price of the 1952 Topps Mantle skyrocketed beyond what anybody had though possible, and at the same time people started paying more attention to rookie cards of star players, initially just established superstars, but also younger stars. Starting in the early 1980s, the term "rookie card" became more and more prominent in the hobby press, and it expanded into popular knowledge later in the decade when the hobby approached the peak of the boom. I remember all this, because I was an active collector starting in the mid-70s, when the concept of a "rookie card" was essentially unknown, and I was still a very active collector in the early 80s when it became ubiquitous. I may post more about this later, with documentation, but that's the basics.