Highlights and Streaks for Girls
A: Done professionally, highlights and coloring can be safe for a 12-year-old. However, I usually encourage parents to discourage their daughters from starting chemical processing of the hair too early, since these steps create permanent changes in the hair and will begin a long period of required maintenance and regular exposure of the hair to chemicals that can damage the hair if not performed properly or not properly cared for afterward.
Unless properly conditioned every day and styled with the hair's vulnerability in mind, the hair can become badly damaged and look unhealthy. There is no good reason to allow a child to have an expensive hair process without knowing that he or she can properly care for it.
If your daughter is mature enough to be able to properly care for her new hair look, then by all means allow her to get color or highlights if you wish to do so. Just remind her before doing so that getting a permanent color or highlighting means that she will either need to have the color/highlights maintained or live with them while they grow out.
You may want to encourage your daughter to try semi-permanent color or low-lighting with semi-permanent color to give her a new look, without needing to make such a long-term commitment to chemical processing.
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See also:
Hair coloring
Highlighting and highlights
Is it ok to dye a child's hair?
Kids and hair decisions
Common-sense tips for kids' hair