My daughter has been begging to dye hair, something like this:
Anyway, I've seen other kids at the school with this ombre style, but the colored portion may be purple, blue, green, or other colors besides the pink shown here, and I think it looks cute. It's not against the school's dress code or anything, either, so I'm willing to let her but my husband says she's too young. Do you think 12 is too young? She'll be 13 next month.
One of my daughter's friends regularly dyes her hair and she's 10. I, personally, have asked my daughter to wait until she is 16 to use any kind of permanent dye. I just don't want her to start down that road of damage at such a young age.
I have sons, but my policy is that they can do whatever they want with their hair as long as it's not insane for school pictures. Hair grows out, goes back to it's "natural" color. No big deal. It's ok for their self-expression. Much rather them do that than get something piereced or tattoed. :)
I don't think she's too young to dye her hair as long as she doesn't use a permanent dye. But as she's going to officially be a teenager next month, maybe you could use the hair color to make a big deal of the occasion. Have a mother/daughter thing to celebrate her being a teenager and dye it then. It will be a "rite of passage of sorts.