Well there are many things they can do with the guns that don't really involve "taking responsibility". The thing is, if a young person does anything, he's going to do it more foolishly than an adult. But I don't think that means we should keep them locked in a box, even if the punishment is going to be lesser for them. Since they do most bad things out of their temporary foolishness, I don't think heavy punishment would be truly effective at deterring these behaviors anyway.
So, you can balance it as a society, although it's never going to be clear cut. That's why we don't let fifteen year olds drive cars. As for guns, I think it would be more sensible to simply require a hunters ed course before they can use a gun.