Many interesting responses, of which I’ll share two (quite different, one from the other):

Jay,
You say, “Wait till you know something, and have some experience of the world, and maybe wait, too, until you have to pay taxes.” By then it will be too late.
You write as though youths who avoid politics will continue straight through to 22 years of age, surrounded by a cordon sanitaire. But leftists who make no distinction between life and politics have authority in schools, libraries, community organizations, etc. Left-liberal inculcation — impressment — begins as early as language comprehension.
In 1988, when I began fifth grade, lessons became noticeably interwoven with leftist cant about such subjects as global warming and acid rain. Pop culture’s infatuation with nihilism coincided with my high-school years. My family tuned in to Rush Limbaugh. Had they not done so, I probably would not have embraced classical-liberal beliefs.
Could I have started politics later? I don’t think so. It would have been too late. My peers believe themselves to be typical, non-ideological Americans, and they espouse and support liberalism because liberalism got to them first.

I had forgotten about acid rain! Used to be a huge concern. Was on the cover of Time magazine every other week, or so it seemed. Big stick with which to beat Reagan, who was said to love acid rain! Oh, that brings back memories.

Anyway, the second letter:

Dear Mr. Nordlinger,
I agree wholeheartedly that children should not hold political signs, etc. It always makes me feel uncomfortable, even if they’re holding a sign supporting something I support, or opposing something I oppose.
I’m also with you on college students, mostly because of my own experience. When I was a student (at a small liberal-arts school), I mindlessly went to anti-Iraq War rallies, called an acquaintance a horrible human being for being a Republican, and voted first for Gore and then Howard Dean. About a year after graduation, I moved to New York City, realized that terrorism is a real threat, began paying taxes in earnest, realized Israel is not the bad guy, reevaluated my idea that “Bush is an idiot who simultaneously is a devious mastermind plotting for oil riches,” re-registered as a Republican, and consider myself a conservative.
Young people are idiots, myself included. Mind you, I’m not saying that Democrats or liberals are automatically idiots. I’m saying that as a college student I identified myself as a Democrat without putting any thought into it. Once I started thinking about it, I realized I wasn’t a liberal. My Republican acquaintance might have been an idiot, too, but was certainly not a horrible human being.
And considering that she was bucking the knee-jerk left-wing tradition, she probably wasn’t an idiot.

No, probably not.