So, if I present my 25-year-old (augh!) high school ID card at a checkpoint --- back from the days when I had a face full of acne, a head full of hair, no mustache, and only one of the two chins I have now --- the TDC should have no problem figuring out that the person on the ID is me, right?

Seriously. People's physical appearance changes over time. If your photograph is on an ID card, you'll gradually look less and less like that photograph, making its usefulness in verifying your identity less and less.

No, you don't magically cease to look like your ID card photo 367 days after its expiration date. But one has to draw a line somewhere. (Do you really want to rely on the judgment of a TDC to say "this doesn't look enough like you anymore?") A year for an expired ID card is a simple enough criterion to enforce consistently.

Of course, this assume that checking ID cards is a useful exercise itself, which is ... well, a discussion for another thread.