Share 'What age for cell phones'

At what age do you think it is appropriate for kids to get a cell phone? I used to be one of those parents that couldn't justify it till they had a lot of extra curricular activities that required they be in contact with me. I finally broke down and bought one for my son on his 13th birthday because his step brother got one from his mom the year before. Now I must say I wish I had done it soo...