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Really Google? I'm 17 years old and you think I'm a little kid who can't handle Google+? So I did what every tech enthusiast would do; I found a friend with an account who is over 18 and got permission to use the account. I got the friend who sent me the invite to send one to the other account. I was excited when I got the invite. But that was short lived when I saw the usual message about being overcapacity.
My message to Google is: your invite system sucks really horribly and you should at least let high schoolers in without making them lie about their age. What's your target demographic? Facebook seems to consist of a lot of high schoolers.