I would never say to ds 'because I said so', if I ever said no to anything I would discuss it and explain my reasoning.
I was on FB and know how it works, it was all chat rooms and yahoo messenger when I was a kid.
I deleted FB for various reasons of my own, nosey people, idiots on news feed, witnessing cyber bullying.
I don't think you need to use something at 12, 13, 14 to be able to handle it at 18, 19, 20. Children are just not responsible enough for certain things, imo, that's why you can't drink, drive, do lots of things until you're 18. Children also just don't need certain things, they see their friends at school everyday, after school, not like adult trying to keep in touch.
Most people are naturally just more mature by 18, by the time they've worked, been to college, you can learn Facebook/Twitter or whatever settings at any age.
For people 30+ the Internet would have barely been around in their teens, doesn't mean they are clueless about Internet safety now does it?