It is usually when you get your DL or ID as far as I know. But, you can do it yourself before I think. But, not sure how that process works. Both my bros are USC so they've had DL since before 18. They received their letter on their 18 birthday.
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Sent 10/19->WAC accepted 10/24
I-797 arrived 10/27
Biometrics letter arrived 10/29 Biometrics done 11/20
EAD: February 7, 2013 SSN:Already Have 
DL:Gonna take a month or two..haha.never bothered to learn..too scurred