The average age for the girl to start using chemical products should be above 12 years old and ONLY if she is capable enough to take care of her hair daily in terms of conditioning, styling and other stuff... she has to keep in mind that, if she starts applying chemical products such as highlighting, coloring, and so on, she has to keep the process up as she has to begin a long period of required maintenance to protect her hair from being damaged…

Therefore, I advise the parents to check over their girls’ abilities in taking daily cares of their hairs… if she is not a mature enough to hold this responsibility, I would definitely discourage her from taking this step, as there is no excuse for letting the girl have an expensive hair process if she is not capable enough to take care of her hair afterward!