Thank you for the question. Acne is common when a child hits puberty and this is the age that acne starts developing. It is not uncommon to see acne starting in children from as young as 10 and 11 years old.
You can certainly start acne treatment for a 12 year old girl. Treating acne early helps to prevent worsening of acne and decreases the risk of developing acne scars when the acne gets worse and helps to improve the self confidence of the child.
Treatment depends on the severity of the acne. In mild cases, acne cleansers and topical treatments like benzoyl peroxide, differin and tretinoin creams or topical antibiotic creams may be sufficient to control the acne.
In more severe cases, oral treatments like oral antibiotics or isotretioin may be needed. Do see your doctor/dermatologist who can assess the severity of your child's acne and advise you on the most suitable treatment options.