Lately I've been wrangling with designers and proofreaders at work over a habit that only someone about my age or older would have: typing two spaces between sentences.
When I learned to type in high school, we used manual typewriters. If you were really lucky, you could use one of the huge IBM Selectric typewriters now and then. Because typewriters produce letters that are equally spaced, the teacher, Mr. Bates (or, as the class clowns found endlessly amusing, Master Bates, ha! ha!), taught us to put two spaces between sentences to make it easier to see the divide.
Little did I know that in 20+ years, this habit would come to signify my advanced age and irritate everyone who handles my copy.
By the time I started working with word processors (WordPerfect for Windows 3.1 - woohoo!), my typing habits were already ingrained. I still type 85+ words per minute, even allowing for the unnecessary spaces.
My wrangling with proofreaders doesn't stop with the sentence spacing. It turns out I've become sentimentally attached to word usage that has now been "modernized" and changed in ways I don't like. For example, I consider "backyard" an adjective and "back yard" a noun (with adjective sidecar). Not so anymore, apparently. Now "backyard" is the all-purpose word and you don't get to use "back yard" at all anymore. This makes me feel all pouty.
What about your habits have you realized shows your age in subtle ways?