Why do papers found online often not say their publication date? I have noticed that it can be hard to figure out how old a paper is from the paper itself. It is not that it is impossible: most of the time it can be found by a Google search. But why is there usually no publication data on the first page of a paper?

Here is an example: http://www.cs.toronto.edu/~fritz/absps/imagenet.pdf
One thing that may be causing the phenomenon that you are seeing is that in fields like computer science, authors are frequently allowed to put preprints online on their website, but not the final "official" version.  Such preprints often do not carry a date stamp, while the official version available from the publisher's website does.