I don't care how old you get...you're never too old for a Nerf gun. Or a squirt gun, but that's different post.
The appeal, of course, is not that it's a gun, where you can work out aggression or make a statement on the person you're shooting, but that it's simply a way to engage your friends or family from across the room. It's like tossing a ball at them, or throwing a grape high in the air for them to catch in their mouth.
And when you're older, then you can do all kinds of fun stuff with 'em, like make this crazy beast.