How old do you have to be to play skate 3 online?

Skate 3 is rated Teen

Content is generally suitable for ages 13 and up however by using EA Services, you affirm that you are at least 18 years of age (or have reached the age of majority if that is not 18 years of age where you live.)

Also, if you are experiencing an error related to your age please read this message (XBox 360 users):
Too young to play - Error

Hope this helps.