My house is about 25 years old - probably the original shower plumbing - in
the five years since I've been in the house, the temperature adjustment knob
for the shower has always had to be turned almost all the way to hot to get
the shower nice and hot. The adjustment knob is a single one-piece knob -
the kind that ya pull out to control the water, and turn left for hot and
right for cold. It's getting to where need to turn it all the way to hot to
get a decent temperature shower - I imagine that means that the mixing valve
is on its way out.

Before I go replacing things that I don't understand, or call a plumber to
look at it, when a mixing valve goes out, does it tend to fail as I've
described, or might it be a problem with the hot water supply line?

Thanks!