[Pseudoscience-is-malarkey (OP)]

Obviously these are not good for one's health and can easily send 'em to an earlier grave if unchecked, but the whole hair thing... Can it actually cause someone's hair to "age" or is this the stuff of fiction, specifically 19th century literary cliches?

[Janus]

Hair turning grey or white is due to the stem cells that produce pigment not doing their job anymore.   This makes the hair translucent.  Whether the result is "grey" hair vs. "white" hair, depends on the hairs that are have pigment.   The translucent hairs can pick up and scatter this.  So, if you are person with dark hair, it will start off looking gray, but as less and less hairs produce pigment, it will begin to look white and whiter.
Some studies have shown that stress can have a detrimental effect on the stem cells that produce the pigment. And once that damage is done, it is permanent.  So yes, there does seem to be a connection between stress and hair greying.

[Petrochemicals]

My auntie lost all of the curl from her hair during bombing in WW2 and went grey because of a family tragedy. I think it maybe a specific trait in some people.

[LaurenaS]

There are evidences that stress may have the potential for a person's hair to turn grey or white faster due to oxidative stress in the hair follicles, but no direct link can be found.