Age is just a number. I've known mature 16 year olds and incredibly immature 21 year olds. I think determining when someone is "grown" is a little more complicated than a simple statement. I think it has to do with a lot of factors, maturity being one of them. I guess a broad description would be someone who's independent and able to control their life with minimal assistance.

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"America prays for God to destroy its enemies. Our enemies pray for God to destroy America. Somebody's going to be disappointed! Somebody's wasting their fucking time! Could it be.... everyone?!"
- George Carlin