I was quite opinionated when I was, say, 9. I hadn't learnt how to effectively argue my opinions yet, but I was perfectly capable of forming them, hearing what others had to say (my parents and teachers often disagreed about things, and my peers definitely did) and making up my own mind. Learning how to express them respectfully, with signs or otherwise, is part of that, and if I hadn't been allowed to, I would not have learnt how to effectively argue them (or change my mind) until much later. So I say, anyone who's old enough to have an opinion ought to be allowed to express it.

And this Prop 8 thing ain't over yet! =)