Your first sentence makes my point: at age 14, politics and the rights & responsibilities of free speech are subjects they're still learning, and not capable of forming a fully informed opinion. Adolescent brains [if that's not an oxymoron, having raised 2 myself, lol] are not capable of adult decisions - yet.
The right to bear arms is as controversial an issue right now as civil rights were in the 60's, or the war in Vietnam. Neither of those issues would raise an eyebrow if a kid wore a shirt that referenced them, today, because they're not topical - but the 2nd amendment certainly is. 10 years ago, or 10 years from now, it wouldn't matter, but right now? It's a political issue.