I ran a survey a few weeks ago for people interested in linguistics. Thanks to the 75 people who had responded as of the writing of this post (if you didn’t fill it out then, feel free to do so now here).
Here are some summary statistics for the data nerds among us. Many more graphs below the cut — click any graph to enlarge.

Current age ranges from 15 to 33, with clusters at 15-16, 18-20 and 21-24, plus a scattering of 25 and older.

I find it interesting that the age-at-first interest is somewhat bimodally distributed: there is an unsurprising peak at age 18, when I assume a lot of people stumble into intro linguistics classes, but there’s also a peak at age 15-16, and fairly high numbers at 13 and 14 as well. Maybe this is an age when people start reading nonfiction related to linguistics? (Steven Pinker’s The Language Instinct, getting lost in wikipedia linguistics articles, etc).