No, Twitter is actually not six years old yet. I am talking about using Twitter when you are only 6 years of age.
Yesterday on French television, the evening news had a segment about a teacher in Dunkirk (Dunkerque for the natives) who is using Twitter in the classroom to teach French to his first-grade pupils.
Jean-Roch Masson, the teacher, explains that you become a good reader by first being a good writer.
"In your next tweet, you need to mention something that made you laugh or something that made you smile" he instructs the class.
I guess the 140 character limit is more than plenty for kids to write simple sentences in just a few words.

And, as the youthful Mr. Masson seems to be technically inclined, kids are asked to come to a large interactive projection screen instead of a chalk-board to explain their own tweets and dissect the tweets from their schoolmates.
"You need to read them, and then respond", says a little girl.
"... and in between you need to think, think and think again."

And then she adds:
"Thinking, that's my cup of tea" ("Reflechir, c'est mon truc").
She made my day. Thinking as a kid's favorite activity. I wish it was also the favorite thing that most adults would do before they actually post a tweet.
Parents were somewhat less enthusiastic. Imagine your 6-year-old coming back from school and asking permission to open a Twitter account. I am not sure if Twitter is actually safer than email or instant messaging. It does prevent strangers from sending a direct message to someone who is not following them already. One more thing for me to think about before I write something...
Anyhow, kudos to Mr. Masson and his class for what seems a great way to get the kids engaged in writing good sentences in French, a language with even more grammar and spelling exceptions than rules. And all this in 140 characters.
"Je twitte, tu twittes, il twitte ..." is how you would conjugate the verb in the present tense in French.
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There is no data to support your assertion that children are dumber. In fact, just the opposite. Children are smarter. Tests are constantly revised to take into account this progress. What you may be confusing is our results in comparison to other countries. Set aside, for a moment, that the data in those comparisons has flaws. There is still the reality that those other countries have grown better faster than we have, not that our children have grown dumber.
To your final thought, there are many ways to protect children online. Using that, we should not allow those same children to walk home from school alone. After all, every abduction, assault, and worse has happened in the real world not on the Internet. And fortunately, the protections available for the Internet are pretty good.
Which is why I think this teacher's initiative is remarkable, because he focuses on making kids good readers and good writers in a controlled environment, using super simple technology.
If you are not doing so yet, use Twitter, and you will realize that strangers cannot reach out to you directly, unlike email. Perhaps a more kid-optimized version with direct parental supervision could be devised (may be this already exists somewhere given the thousands of application developers around there).
Broad-based assertions that technology makes every kid dumber are not justified.