Long time reader, first time poster. I had similar concerns, but used Sudoku.

As long as you are doing a good number of puzzles on a regular basis, your personal results can yield good trend data. You can try averaging the result of every 5 scores to creat a sample set. Helps smooth out and normalize the data. See Statistical Control Charts for more ways to look at the data.

Comparison to your cousin probably won’t mean much, as it sounds like your on two different learning curves (your consistent performance versus their improvement). If you had scores from others to factor in as well, that would make your performance compared to the group a more valid measure.

It’s nice to have the data to go back to. I don’t think crossword, or sudoku really means much to how well your brain performs, but believe that a change in the trend of you performance would point to some cognitive decline. Crossword, Sudoku, reading, even paint is a good way to track cognitive ability. Saw a series of self portraits by an artist William Utermohlen done after he was diagnosed with Alzheimer’s. We focus a lot on physical ability as we age as you feel it with every movement, but mental performance changes are a lot more subtle.