Gotta hand it to her. She stays young because she's one of, if not the most, versatile puzzle out there. The basic rules are simple: here's the clues for all the entries going across, and there's the clues for all the down words. That's it. So astonishingly simple, it's hard to believe she hasn't been around forever.Yet within these very strict rules, a whole vast universe of variety exists. (Sidebar, I believe that the reason sudoku remains so successful is that it too shares the once-you've-heard-the-oh-so-easy-to-understand-rules-you-can-solve-every-one-of-them parameters.)
What's also intriguing is that the basic list of "rules" involving construciton were set in place fairly quickly. And despite the fact that trailblazing puzzlemakers have tried to modernize the puzzle every decade, the original rules remain sacrosanct.
The first-ever wave crossword-mania came in the 1920s. They popped up everywhere (much like sudoku a couple years back), and the public's thirst couldn't be quenched. The craze was so out of hand that editors were swamped with so many crappy submissions, they simply didn't have time to wade through the slush pile. It was Margaret Farrar (originally an editor for Simon & Schuster, later the first puzzle editor for The New York Times) who came up with the basic rules for American crosswords to weed out the grids that she deemed were too easy to make:
1. The grid has an odd number of squares on both sides, and the black and white pattern has 180° symmetry (that is, the grid looks the same upside down as it does right-side up).
2. Every letter has to be checked (that is, every letter belongs to both an Across and Down entry).
3. No more than 1/6th of the grid can be black squares.
4. The smallest words must be at least three letters long.
Serious constructors still abide to these rules today. That isn't to say that some aren't broken. The New York Sun, for instance runs 16x15 grids, even wide-wide-open asymmetrical grids on occasion, when the theme warrants. Even I broke the rules with a left/right symmetry grid with the David Cross quip puzzle. But those are exceptions are rare.
The crossword seems to be in good shape today -- she's in the middle of yet another renaissance today, thanks to The Internet and computers. Technology nowadays has allowed for computer-assisted WTFF? grids that would have been nearly impossible ten years ago. Every newspaper on the planet has their crossword available to solve on the Internet (as well as many other specialty sites like this one). And the act of Googling has now become part and parcel to the solving experience.
Here's to another (at least) 95 years!
Tangent: I was going to do a spiel about hard vs. easy puzzles, but that will have to wait to another day. Just wanted to throw this out there: one of my test-solvers, Mike Nothnagel, got the theme from this one just by reading the clues. He does that from time to time. Kinda pisses me off.