Posted By:
Hal LewisAs Gil points out, there are a LOT of problems with "rookie" cards -- both in the old days AND in the modern days.
There have been MINOR league cards as far back as the N172 Old Judges... right throught the Zeenuts... and all the way up until today.
SOME people collect these as rookie cards... but on this board, we lovingly call them "PROOKIES" (which stands for Pre-Rookie). The player was not pictured on a major league team, so most true rookie collectors do not collect these. These are sought after by the "Earliest Image" collectors.
Also, as you point out, there have been situations like O'Rourke and Musial and others for over a century. These are affectionately known as "FROOKIES" (which stands for First Card but NOT in first season of Major League play).
FROOKIES are "better" than PROOKIES, as most ROOKIE card collectors will also need a FROOKIE to finish their collection. It is still the earliest Major League card of the player... so it is the closest thing to a true rookie card available for some players.
There are other problems as well... especially now that EXHIBIT CARDS and POSTCARDS seem to be prevalent.
For example, one set of Exhibit Postcards is now labeled as being issued from 1926-1929 in the Standard Catalog.
If a player was a rookie in 1926... how can you really know whether the Exhibit was issued in 1926 or in 1929?
This really complicates things when there is a different card of the same player that DEFINITELY came out in 1927 or 1928. Which is earlier??
I say you get BOTH and then can't go wrong. 
There is also the debate over whether or not cards that were issued in the SAME YEAR are ALL rookie cards... or ONLY the very FIRST set if it can be proven as to which was first??
If Leaf put out a set in January of 1949... and Bowman put out their set in July of 1949... but both sets feature the same players on their 1949 teams... can they BOTH be rookie cards?
I say YES.
Otherwise, there is NO WAY to EVER tell whether Topps beat Fleer to the market in any given year, etc. WAY TOO COMPLICATED and not relevant.
Then there are a LOT of other issues that are more your PERSONAL preference.
Do postcards or exhibit cards even "count" as baseball cards? Do strip cards count? Do team cards count?
Don't drive yourself crazy... and in the end...
CWYWC !!!!