Several employees took the stand Monday to testify that Kirkpatrick, who was much older than them, was often testy and impatient.There is an invisible verb hiding at the end of one clause there--an implied verb, I believe they call it--and once it reveals itself, the error becomes strikingly apparent. You wouldn't say, "Kirkpatrick, who was much older than them were, was often..." Rather, it should be, "who was much older than they," or, if that sounds too Charles Emerson Winchester for you, you can make the implicit verb explicit and say "who was much older than they were..."
Sure, you can make the claim that "older than them" has gained colloquial currency, but remember there are instances where the difference between the objective and the subjective pronoun makes a world of difference in meaning.
For instance, if I say "I hate Ben Affleck more than her" I could be trying to indicate that I have a lower regard for Ben Affleck's "talent" than my wife does. But it could also mean that I hate Ben Affleck more than I hate Kim, when the fact is I don't hate Kim at all--even when she puts my iPod in the washing machine. However, when I say, "I hate Ben Affleck more than she (does)," there is no mistaking who is being hated, and who is doing the hating.
* "I could see that, if not actually disgruntled, he was far from being gruntled."--P.G. Wodehouse.