I find it somewhat ironic that the entertainment industry glorifies guns on tv and in movies, but yet these same people condemn them at the end of the day.
Growing up, my dad owned a few hunting rifles and took me shooting. This was after i gained a huge interest from none other than, you guessed it, movies and television. I don't specifically remember being taught in school that guns were bad per say, but i was like any other young boy, i enjoyed playing war and cops and robbers and whatever bullshit you do when you're young. My parents didn't discourage it, so here i am today.
I would think the parents would have the biggest influence. If you grow up in a pro gun, or even a neutral house, you're far more likely to grow up feeling the same way. If you are never exposed to guns, and told time and time again they are bad, you're probably far more likely to be anti-gun when you become an adult.
That's my theory anyways. Of the friends i have that are "pro-gun" they grew up around them, or were exposed to them. The people i know that are either anti-gun or indifferent, were never exposed to them.